Method and apparatus for evaluating and optimizing a signaling system

ABSTRACT

A method and apparatus for evaluating and optimizing a signaling system is described. A pattern of test information is generated in a transmit circuit of the system and is transmitted to a receive circuit. A similar pattern of information is generated in the receive circuit and used as a reference. The receive circuit compares the patterns. Any differences between the patterns are observable. In one embodiment, a linear feedback shift register (LFSR) is implemented to produce patterns. An embodiment of the present disclosure may be practiced with various types of signaling systems, including those with single-ended signals and those with differential signals. An embodiment of the present disclosure may be applied to systems communicating a single bit of information on a single conductor at a given time and to systems communicating multiple bits of information on a single conductor simultaneously.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 12/815,382, filed Jun. 14, 2010, which is a continuation ofU.S. patent application Ser. No. 11/686,706, filed Mar. 15, 2007 (nowabandoned), which is a continuation of U.S. patent application Ser. No.11/559,111, filed Nov. 13, 2006 (now U.S. Pat. No. 7,490,275), which isa continuation of U.S. patent application Ser. No. 09/976,170, filedOct. 12, 2001 (now U.S. Pat. No. 7,137,048), which is acontinuation-in-part of U.S. patent application Ser. No. 09/776,550,filed Feb. 2, 2001 (now U.S. Pat. No. 6,873,939), each of which ishereby incorporated by reference herein in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to communication systems and,more specifically, to in-situ testing of communications systems.

BACKGROUND OF THE DISCLOSURE

For information communication and processing systems to operatereliably, it is important to be able to test these systems and measurevarious performance characteristics that pertain to them. Classically,it has been very difficult to observe the fidelity of a signaling systemfrom a transmit circuit to a receive circuit, including a medium throughwhich the transmit circuit is coupled to the receive circuit. It hasbeen especially difficult to obtain in-situ measurements of theoperation of the system. Rather, external test equipment is typicallyintroduced into the system for the purpose of obtaining measurements. Itis common for an external signal generator, for example one capable ofproducing test signals with ultrafast or adjustable transition times,and an external measurement device, such as an oscilloscope, to beconnected to a system under test. However, since such external testequipment has characteristics different from system under test,measurements derived using the external test equipment may notaccurately reflect the actual performance of the system under test.

While it was possible to obtain meaningful information from simplersystems of the past using external test equipment, the increasingcomplexity and operating frequencies of modern systems introduceadditional complications that impair the effectiveness of testing usingexternal test equipment. For example, much higher frequencies andcontrolled impedances make it much harder to introduce external testequipment without distorting the signals being measured and, therefore,affecting the measurements themselves. Moreover, connection anddisconnection of the external test equipment requires time, effort, and,potentially, additional design considerations, such as the provision oftest points within a system. Also, external test equipment does notallow testing to be performed from the perspective of the actual receivecircuit within the system. Thus, such testing cannot definitivelyprovide information as to what the receive circuit actually receives.Therefore, traditional testing techniques fail to provide complete andaccurate information about the system under test. Thus, a technique isneeded to provide complete and accurate information about a system undertest and to enable in-situ testing of the system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a signaling system in accordancewith an embodiment of the present disclosure.

FIG. 2 is a logic diagram illustrating a prior art transmit circuit.

FIG. 3 is a logic diagram illustrating a prior art receive circuit.

FIG. 4 is a logic diagram illustrating a transmit circuit capable ofoperating in a pseudo-random bit sequence (PRBS) test mode in accordancewith an embodiment of the present disclosure.

FIG. 5 is a logic diagram illustrating a receive circuit capable ofoperating in a PRBS test mode in accordance with an embodiment of thepresent disclosure.

FIG. 6 is a logic diagram illustrating a transmit circuit capable ofoperating in a PRBS test mode and a roll test mode in accordance with anembodiment of the present disclosure.

FIG. 7 is a logic diagram illustrating a receive circuit capable ofoperating in a PRBS test mode and a roll test mode in accordance with anembodiment of the present disclosure.

FIG. 8 is a logic diagram illustrating a prior art quad signaling leveltransmit circuit.

FIG. 9 is a logic diagram illustrating a quad signaling level transmitcircuit capable of operating in a PRBS test mode in accordance with anembodiment of the present disclosure.

FIG. 10 is a logic diagram illustrating a quad signaling level transmitcircuit capable of operating in a PRBS mode and a roll test mode inaccordance with an embodiment of the present disclosure.

FIG. 11 is a logic diagram illustrating a prior art quad signaling levelreceive circuit.

FIG. 12 is a logic diagram illustrating a quad signaling level receivecircuit capable of operating in a PRBS test mode in accordance with anembodiment of the present disclosure.

FIG. 13 is a logic diagram illustrating a quad signaling level receivecircuit capable of operating in a PRBS test mode and a roll test mode inaccordance with an embodiment of the present disclosure.

FIG. 14 is a timing diagram illustrating signals in accordance with anembodiment of the present disclosure.

FIG. 15 is a block diagram illustrating a differential receiver that maybe used in conjunction with and embodiment of the present disclosure.

FIG. 16 is a waveform diagram illustrating a differential signal thatmay be used in conjunction with an embodiment of the present disclosure.

FIG. 17 is a waveform diagram illustrating a differential signal thatmay be used in conjunction with an embodiment of the present disclosure.

FIG. 18 is a waveform diagram illustrating a differential signal thatmay be used in conjunction with an embodiment of the present disclosure.

FIG. 19 is a schematic diagram illustrating an example of an input stageof an offsetable differential receiver in accordance with an embodimentof the present disclosure.

FIG. 20 is a schematic diagram illustrating an example of an input stageof an offsetable differential receiver in accordance with an embodimentof the present disclosure.

FIG. 21 is a two-dimensional graphical diagram illustrating arelationship between a waveform of a signal received at the receivecircuit and variations occurring in the interpretation of datarepresented by the signal in accordance with an embodiment of thepresent disclosure.

FIG. 22 is a two-dimensional graphical diagram illustrating theaccumulation of locations of regions where variations are observed inaccordance with an embodiment of the present disclosure.

FIG. 23 is a two-dimensional graphical diagram illustrating theaccumulation of locations of regions where variations are observed basedon sampling pertaining to a differential signal in accordance with anembodiment of the present disclosure.

FIGS. 24A and 24B are a flow diagram illustrating a method in accordancewith an embodiment of the present disclosure.

FIG. 25 is a waveform diagram illustrating an example of a persistentdisplay of overlapping samples of a waveform in accordance with anembodiment of the present disclosure.

FIG. 26 is a waveform diagram illustrating an example of a display ofsamples of a waveform in accordance with an embodiment of the presentdisclosure.

FIG. 27 is a waveform diagram illustrating an example of a persistentdisplay of overlapping samples of a waveform in accordance with anembodiment of the present disclosure.

FIG. 28 is a waveform diagram illustrating an example of a persistentdisplay of overlapping samples of a waveform in accordance with anembodiment of the present disclosure.

FIG. 29 is a waveform diagram illustrating an example of a persistentdisplay of overlapping samples of a waveform in accordance with anembodiment of the present disclosure.

FIG. 30 is a waveform diagram illustrating an example of a persistentdisplay of overlapping samples of a waveform in accordance with anembodiment of the present disclosure.

FIG. 31 is a block diagram illustrating a system in accordance with anembodiment of the present disclosure.

FIG. 32 is a flow diagram illustrating a method in accordance with anembodiment of the present disclosure.

FIG. 33 is a waveform diagram illustrating optimization of crosstalkcancellation resulting from iterative application of the methodillustrated in FIG. 32 in accordance with an embodiment of the presentdisclosure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

A method and apparatus for evaluating and optimizing a signaling systemis described. Evaluation is accomplished using the same circuitsactually involved in normal operation of the signaling system. Suchcircuits are adapted to provide testing capability with minimaladditional complexity. Thus, capability for in-situ testing of asignaling system is provided, and information may be obtained from theactual perspective of a receive circuit in the system. Both the need forthe introduction of external test equipment and the inaccuracy caused byits introduction are avoided. An embodiment of the present disclosuremay be implemented to provide a built-in self-test (BIST) capabilitywithin an operational system. Such capability can be very beneficial,especially where access to internal components of a system wouldotherwise be difficult. For example, in addition to being applicable tosignaling systems where the transmit circuit is located separately fromthe receive circuit, an embodiment of the present disclosure may beapplied to a relatively closed system, for example, a system fabricatedas an integrated circuit. Thus, even when an integrated circuit ispackaged, extensive internal evaluation and optimization capability maybe provided in accordance with the present disclosure.

A pattern of test information is generated in a transmit circuit of thesystem and is transmitted to a receive circuit. A similar pattern ofinformation is generated in the receive circuit and used as a reference.The receive circuit receives the pattern of test information generatedin the transmit circuit and compares it to the pattern of informationgenerated in the receive circuit. Any differences between the patternsare observable. Preferably, the patterns are repeating patterns thatallow many iterations of testing to be performed. Thus, even events thatoccur infrequently within the system may be observed.

In one embodiment, a linear feedback shift register (LFSR) isimplemented to produce patterns. Information obtained from testing maybe used to assess the effects of various system parameters, includingbut not limited to output current, crosstalk cancellation coefficients,and self-equalization coefficients, and system parameters may beadjusted to optimize system performance. Embodiments of the presentdisclosure may be used in assessing a wide variety of phenomena,including, but not limited to, overshoot, undershoot, edge rates,crosstalk, duty cycle error (including the cumulative duty cycle erroracross the transmit circuit, the medium, and the receive circuit),impedance continuity/discontinuity, and the effectiveness of differentreceive and transmit effects. An embodiment of the present disclosuremay be practiced with various types of signaling systems, includingthose with single-ended signals and those with differential signals. Anembodiment of the present disclosure may be applied to systemscommunicating a single bit of information on a single conductor at agiven time and to systems communicating multiple bits of information ona single conductor simultaneously.

Several embodiments of the present disclosure described herein areparticularly useful for obtaining data expressing attributes ofwaveforms communicated from the transmit circuit to the receive circuit.As a brief example that is described in greater detail with reference tothe drawings, the following process is performed: a transmit repeatingpattern is initially transmitted to the receive circuit, one or moreparameters affecting reception of the transmit repeating pattern in thereceive circuit are set, the receive circuit is placed in a mode whereinthe receive circuit generates a receive repeating pattern, the transmitrepeating pattern is repeatedly transmitted to the receive circuit, anyvariation in the relationship between the transmit repeating pattern asreceived by the receive circuit and the receive repeating pattern isdetected, the values of the parameters in effect during such variationare stored, and the process (or a portion thereof) is repeated fordifferent values of the parameters.

Variations detected in such a process occur when the relationshipbetween the transmit repeating pattern as received by the receivecircuit and the receive repeating pattern does not remain fixed overmultiple iterations of the patterns. Since, logically, the relationshipbetween the patterns is known to be fixed, such variations indicatefailures involving uncertainty in the communication of the transmitrepeating pattern to the receive circuit. Such failures can occur whenthe parameters affecting reception of the transmit repeating pattern inthe receive circuit are set to values that border on ranges of values ofthe parameters that provide for accurate communication of the transmitrepeating pattern to the receive circuit.

When the parameters are set to values within the range of values thatprovide for accurate communication of the transmit repeating pattern tothe receive circuit, accurate communication is reliably provided. Thus,no variations are detected in the relationship between the transmitrepeating pattern as received by the receive circuit and the receiverepeating pattern. When the parameters are set to values well outsidethe range of values that provide for accurate communication of thetransmit repeating pattern to the receive circuit, the transmitrepeating pattern is consistently misinterpreted by the receive circuit,thereby yielding a relationship between the transmit repeating patternas received by the receive circuit and the receive repeating patternthat does not vary. Thus, no variation is detected. Then, therelationship between the transmit repeating pattern and the receiverepeating pattern may be said to exhibit repeatability. However, whenthe parameters are set to critical values bordering the ranges of valuesthat provide for accurate communication, the transmit repeating patternwill sometimes be accurately interpreted by the receive circuit, but, atother times, the transmit repeating pattern will be inaccuratelyinterpreted by the receive circuit. Thus, variation will occur and willbe detected. Then, the relationship between the transmit repeatingpattern and the receive repeating pattern may be said to exhibitnon-repeatability. Such a phenomenon can occur when parameters such as atiming margin (e.g., setup and hold times) or a voltage margin are setto values insufficient to provide reliably accurate communication. Aparameter affecting the voltage margin may be set by setting a voltagereference used for the comparison of voltages of a signal representingthe transmit repeating pattern being received at the receive circuit.

By performing such testing over multiple iterations over large numbersof combinations of parameter values, the subset of those combinationswhere variations occur can be identified. If each parameter isrepresented by a dimension in an n-dimensional space (for n parameters),the subset of the combinations where variations occur can be mapped intothat n-dimensional space. For example, using two parameters (e.g.,timing margin and voltage margin), a two-dimensional mapping can beprovided. For typical receive circuit components, variations tend to beobserved across contiguous combinations of parameter values. Thus, sucha two-dimensional mapping typically exhibits one or more lines orcontinuous curves formed from adjacent points representing combinationsof parameter values yielding variations. The thickness or width of suchlines or curves depends on the number of critical values of parametersfor which variations are observed, especially when several values of oneparameter yield variation for a fixed value of another parameter.

For relatively thin or narrow lines or curves, the mapping approximatesa trace of the transmit repeating pattern as received at the receivecircuit of the sort that one having experience using an oscilloscopewould perceive as familiar. Thus, embodiments of the present disclosuremay be used to display an eye diagram of multiple iterations of thetransmit repeating pattern, or, by altering the timing of the transmitrepeating pattern and the receive repeating pattern as multiples orsub-multiples of one another, the trace can be “unfolded” to yield adisplay without overlap of different aspects of the waveform representedby the mapping, thereby allowing finer details of the waveform to beobserved. Therefore, embodiments of the present disclosure may be usedto provide an in-situ “virtual oscilloscope” capability for observingrepresentations of signals while avoiding external influences on systembehavior typically introduced when traditional test instruments arecoupled to a system under test.

FIG. 1 is a block diagram illustrating a signaling system in accordancewith an embodiment of the present disclosure. The system comprisestransmit circuit 101, medium 102, and receive circuit 103. Transmitcircuit 101 comprises shift register 105 and test loop 106. Transmitcircuit 101 can operate in a normal mode or a test mode. In the normalmode, shift register 105 receives data via a data input 104, which maybe a parallel or serial data input. Shift register 105 shifts the bitsof the data to the right, providing each bit as a output at theinterface with medium 102.

The data are transmitted across medium 102 to receive circuit 103. Inaddition to providing output to medium 102, shift register 105 alsoprovides its output to test loop 106. Test loop 106 provides an input107 to shift register 105. In the test mode, shift register 105 and testloop 106 function to generate a test pattern for transmission overmedium 102. The test pattern is preferably, although not necessarily arepeating pattern. Test loop 106 may be a simple loop, such as a loop ofwire, or it may include other combinational and/or sequential logic. Forexample, it may include logic gates such as AND, OR, NAND, NOR, XOR,and/or XNOR gates. Such gates may be used to implement an LFSR. An LFSRis capable of producing maximal-length repeating patterns with a minimumof additional complexity. The LFSR can produce pseudo-random bitsequences (PRBS), which may be beneficially applied to testing thesystem under a wide variety of logical conditions. When an LFSR isimplemented, test loop 106 serves as a transmit linear feedback logicgate and shift register 105 serves as a transmit shift register.Coupling 119, which couples shift register 105, test loop 106, and,optionally, transmitter 115 or medium 102, serves as transmit shiftregister output. Coupling 107 serves as a transmit shift register inputwhen the transmit circuit is operating in a test mode. When an LFSR isimplemented, additional transmit shift register outputs can be providedfrom shift register 105 to additional transmit linear feedback logicinputs of test loop 106.

Receive circuit 103 includes shift register 108 and test loop 111. Inthe normal mode, receive circuit 103 receives data from the transmitcircuit 101 via medium 102, which may, as one example, be an electricalconductor coupling the transmit circuit 101 to the receive circuit 103.The bits of the data are shifted through shift register 108 and providedat data output 109. When an LFSR is implemented, test loop 111 serves asa receive linear feedback logic gate and shift register 108 serves as areceive shift register. Coupling 110, which couples shift register 108to test loop 111 serves as receive shift register output. Coupling 112serves as a receive shift register input when the receive circuit isoperating in a test mode. When an LFSR is implemented, additionalreceive shift register outputs can be provided from shift register 108to additional receive linear feedback logic inputs of test loop 111.

As appropriate, the transmit circuit 101 may include a transmitter 115coupled to medium 102 and the receive circuit 103 may include a receiver116 coupled to the medium 102. In that case, an output of shift register105 is coupled to an input of transmitter 115 and to an input of testloop 106. The output of transmitter 115 is coupled to medium 102. Medium102 is coupled to an input of receiver 116. A receive clock signal isprovided at input 117 of receiver 116. A voltage reference signal isprovided at input 118 of receiver 116. Both the receive clock signal atinput 117 and the voltage reference signal may be varied over a widerange to allow testing of the system under a wide variety of conditions.

Transmitter 115 may be any suitable transmitter. An example includes,but is not limited to, a driver circuit for driving signals onto medium102. The driver circuit may provide desirable characteristics, forexample, a controlled source impedance and well-defined transitiontimes. Transmitter 115 may provide a single-ended signal or adifferential signal over medium 102. Transmitter 115 may be able tocommunicate one or more bits of information over a single conductorsimultaneously.

Likewise, receiver 116 may be any suitable receiver. An exampleincludes, but is not limited to, a receiver for determining digitalsignaling levels of signals present on medium 102. Receiver 116 mayprovide desirable characteristics, for example, one or more voltage orcurrent level thresholds, controlled hysteresis, and controlled timing.Receiver 116 may be configured to receive a single-ended signal or adifferential signal from medium 102. Receiver 116 may be able to receiveone or more bits of information over a single conductor simultaneously.

While subsequent Figures, for example, FIGS. 4, 6, 9, and 10 areillustrated without a transmitter such as transmitter 115, theembodiments of these Figures may be implemented with such a transmitter.While subsequent Figures, for example, FIGS. 5, 7, 12, and 13 areillustrated without a receiver such as receiver 116, the embodiments ofthese Figures may be implemented with such a receiver.

Shift register 108 is coupled to test loop 111 via coupling 110. Theoutput of the test loop 111 is coupled back to an input of shiftregister 108 and also to an input of comparison circuit 113, which may,as one example, be implemented using an XOR logic gate. Another input ofthe comparison circuit 113 is coupled to medium 102 and receives thetest pattern transmitted by transmit circuit 101 over medium 102.Comparison circuit 113 provides a comparison output 114. In the testmode, shift register 108 and test loop 111 operate to produce a patternidentical to or deterministically related to the pattern produced bytransmit circuit 101. Comparison circuit 113 compares the patterngenerated in the transmit circuit 101 to the pattern generated in thereceive circuit 103 to determine for each bit whether the patternsmatch. To establish a relationship between the pattern produced by thetransmit circuit 101 and the pattern produced by the receive circuit103, information to synchronize the pattern of the receive circuit 103with the pattern of the transmit circuit 101 may be communicated fromthe transmit circuit 101 to the receive circuit 103, or elements of thereceive circuit may be preloaded with such information.

The system may be operated in roll test mode. In the roll test mode, thepatterns need not necessarily match, as long as they are repeatable. Inthe roll test mode, the system operates as a “repeatability detector.”The roll test mode generates repeating patterns, and, upon comparison,any variation from the repeating patterns is detected. Thus, the patterngenerated by the transmit circuit and the pattern generated by thereceive circuit need not be the same. In fact, the receive circuit isnot preloaded with information to synchronize its pattern with that ofthe transmit circuit. Rather, the receive circuit is seeded by thetransmit circuit.

In the roll test mode, the timing of receiver 116 may be adjusted, forexample, swept over a range, by varying the receive clock signal at node117, and the symbol thresholds of receiver 116 may be adjusted, forexample, swept over a range, by varying the voltage reference signal atnode 118. While the voltage reference signal is stated in terms ofvoltage, it may be implemented to allow adjustment of other electricalparameters relating to reception of signals at receiver 116.

Shift registers with which embodiments of the present disclosure may beimplemented, for example, shift registers 105 and 108, preferablycomprise a plurality of registers coupled to one another. As an example,such registers are preferably implemented using D flip flops with the Qoutput of one D flip flop coupled to the D input of a subsequent D flipflop. The D flip flops of a shift register are preferably clocked inunison with one another.

Several signaling systems such as that illustrated in FIG. 1 may be usedtogether. For example, a transmit circuit of a first signaling systemmay be used to produce a pattern, and a receive circuit of a secondsignaling system may be used to observe any influence of the pattern ofthe first signaling system on the second signaling system. Such atechnique is useful, for example, to observe crosstalk that may occurbetween the signaling systems. Since crosstalk can occur without anintentional connection between the signaling systems, no such connectionneeds to be made between the signaling systems, and the media of thesignaling systems may be electrically insulated from one another.

In the embodiment illustrated in FIG. 1, shift register 105 and testloop 106 form a transmit repeating pattern generator capable ofproducing a repeating pattern signal. Data input 104 serves as atransmit data input, and medium 102 provides a transmit data output forthe transmit circuit 101 and a receive data input for the receivecircuit 103. The transmit circuit 101 produces a transmit data outputsignal at the transmit data output based on a transmit data input signalobtained from the transmit data input when the transmit circuit isoperating in a normal mode. When the transmit circuit 101 is operatingin a test mode, the transmit circuit 101 produces a transmit data outputsignal at the transmit data output based on repeating pattern signal.

The receive circuit 103 is operably coupled to the transmit circuit 101via medium 102. The receive circuit 103 receives the transmit dataoutput signal from the transmit circuit at the receive data input. Theshift register 108 and the test loop 111 form a receive repeatingpattern generator capable of producing a receive repeating pattern,which may or may not be the same pattern as the repeating pattern signalproduced by the transmit circuit. Data output 109 serves as a receivedata output. When the receive circuit 103 is operating in the normalmode, the receive circuit 103 produces a receive data output signal atthe receive data output based on the transmit data output signal. Whenthe receive circuit 103 is operating in the test mode, the receivecircuit 103 produces a comparison signal based on comparison dependenton the transmit data output signal and the receive repeating patternsignal.

At this point it should be noted that the transmit repeating pattern maybe received in a test receiver (not shown) separate from the receivecircuit 103 when the transmit circuit 101 is operating in a test mode.Also, the transmit repeating pattern may be transmitted from a testtransmitter (not shown) separate from the transmit circuit 101 when thereceive circuit 103 is operating in a test mode.

The shift register 105 of the transmit circuit 101 may be thought of asa transmit data storage element. Alternatively, the transmit datastorage element may be implemented using another structure capable ofstoring data and allowing the sequential transmission of the data, forexample, when the transmit circuit 101 is operated in a normal mode. Inthe test mode, the transmit data storage element is capable of providinga repeating pattern signal, wherein the transmit circuit 101sequentially transmits the transmit data output signal based on therepeating pattern signal. The repeating pattern signal may represent asequence of data bits, with the transmit data storage element storingeach of the data bits, or the repeating pattern signal may have a datalength greater than the data capacity of the transmit data storageelement. For example, a LFSR may be used to produce a repeating patternsignal having a data length (e.g., a number of bits produced as therepeating pattern signal before the repeating pattern signal beginsrepeating) much greater than the data capacity of the transmit datastorage element (e.g., the number of bits that can be stored in thetransmit data storage element).

The transmit data storage element may be divided into transmit datastorage sub-elements during operation in the normal mode. For example,odd-numbered bits may be handled by one sub-element (e.g., one pipelinestructure), while even-numbered bits may be handled by anothersub-element (e.g., another pipeline structure). Thus, distinct data arepassed through each of the plurality of sub-elements when the transmitcircuit is operating in the normal mode. Since such sub-elements may berather short, they might not yield a repeating pattern signal having asufficiently high level of desired entropy in the test mode. Thus, thetransmit data storage sub-elements may be united into a single unit inthe form of the transmit data storage element for providing therepeating pattern signal when the transmit circuit is operating in thetest mode. Thus, a much “richer” repeating pattern signal exhibitingsubstantially higher entropy may be provided. Alternatively, if higherentropy is not needed, the longer data length of the repeating patternsignal made possible by such union may be used to specify a longerspecific bit sequence for the repeating pattern signal.

The data storage element may be loaded from the transmit data input toinitialize the test mode from a source other than the transmit datainput, for example, the transmit data storage element may be loaded viaa parallel transmit load input.

In the receive circuit 103, the shift register 108 may be thought of asa receive data storage element. Alternatively, the receive data storageelement may be implemented using another structure capable of storinginformation relating to either a receive data input signal or arepeating pattern signal. In the normal mode, the receive data storageelement outputs a receive data output signal based on a receive datainput signal received at the receive data input. In the test mode, thereceive data storage element provides the repeating pattern signal. Therepeating pattern signal may represent a sequence of data bits, with thereceive data storage element storing each of the data bits, or therepeating pattern signal may have a data length greater than the datacapacity of the receive data storage element. For example, a LFSR may beused to produce a repeating pattern signal having a data length (e.g., anumber of bits produced as the repeating pattern signal before therepeating pattern signal begins repeating) much greater than the datacapacity of the receive data storage element (e.g., the number of bitsthat can be stored in the receive data storage element).

The comparison circuit 113 serves as a comparison element, performing acomparison of a relationship between the repeating pattern signal andthe receive data input signal received at the receive data input toproduce a comparison output signal based on the comparison when thereceive circuit 103 is operating in the test mode.

As with the transmit data storage element, the receive data storageelement may be divided into receive data storage sub-elements (e.g.,pipeline structures) during operation in the normal mode. Thus, distinctdata are passed through each of the plurality of sub-elements when thereceive circuit is operating in the normal mode. The receive datastorage sub-elements may be united into a single unit in the form of thereceive data storage element for providing the repeating pattern signalwhen the receive circuit is operating in the test mode.

The receive data storage element may be loaded from the receive datainput to initialize the test mode from a source other than the receivedata input, for example, the receive data storage element may be loadedvia a parallel receive load input.

An embodiment of the present disclosure may be implemented in a mannerso as not to be incompatible with an existing transmit circuit, forexample, the transmit circuit of FIG. 2, and/or an existing receivecircuit, for example, the receive circuit of FIG. 3. Such implementationof an embodiment of the present disclosure can be used to overcome thedisadvantages of the existing circuits.

FIG. 14 is a timing diagram illustrating signals in accordance with anembodiment of the present disclosure. A transmit circuit load signal1401, a transmit circuit test mode signal 1402, a receive circuit testmode signal 1405, and a comparison output signal 1406 are illustrated. Aplurality of transmit circuit data 1403 and a plurality of receivecircuit data 1404 are also illustrated.

A PRBS test mode is entered upon the assertion 1409 of the transmitcircuit test mode signal 1402. Data are loaded into a shift register inthe transmit circuit on the rising edge 1407 of pulse 1408 of thetransmit circuit load signal 1401. Sufficient transmit circuit data 1403to initialize the transmit circuit and the receive circuit to likestates is communicated from the transmit circuit to the receive circuit,where it appears as receive circuit data 1404. Bits 1413, 1414, 1415,1416, 1417, 1418, and 1419 of transmit circuit data 1403 arecommunicated to provide bits 1420, 1421, 1422, 1423, 1424, 1425, and1426 of receive circuit data 1404 respectively. These data arecommunicated between time 1411 and time 1412, during period 1410, andserves to seed the receive circuit with appropriate data.

Once sufficient data has been communicated between the transmit circuitand the receive circuit, receive circuit test mode signal 1405 isasserted at assertion 1427. Then, between time 1429 and time 1430,during period 1428, testing may be performed using the seeded receivecircuit data. If, during the testing, an element of transmit circuitdata being transmitted to the receive circuit does not match acorresponding element of receive circuit data, comparison output signal1406 is asserted, such as occurs at assertions 1433 and 1434, whichoccur during period 1432 after time 1431. When testing is completed, thesystem may be returned to its normal mode by deasserting the transmitcircuit test mode signal 1402 and the receive circuit test mode signal1405.

In a transmit circuit capable of a PRBS test mode, a PRBS test signal ofthe transmit circuit may be asserted to serve as the transmit circuittest mode signal 1402. In a transmit circuit capable of a roll testmode, a roll test signal of the transmit circuit may be asserted toserve as the transmit circuit test mode signal 1402. In a receivecircuit capable of a PRBS test mode, a PRBS test signal of the receivecircuit may be asserted to serve as the receive circuit test mode signal1405. In a receive circuit capable of a roll test mode, a roll testsignal of the receive circuit may be asserted to serve as the receivecircuit test mode signal 1405.

FIG. 15 is a block diagram illustrating a differential receiver that maybe used in conjunction with an embodiment of the present disclosure.Differential receiver 1501 comprises a non-inverting input 1502, aninverting input 1503, an offset input 1504, and an output 1505. Offsetinput 1504 may be implemented in various ways, for example, as asingle-ended input or a differential input.

FIG. 16 is a waveform diagram illustrating a differential signal thatmay be used in conjunction with an embodiment of the present disclosure.The differential signal comprises a signal 1601 and its complementarysignal 1602.

FIG. 17 is a waveform diagram illustrating a differential signal thatmay be used in conjunction with an embodiment of the present disclosure.The differential signal comprises a signal 1701 and its complementarysignal 1702. The signal 1701 and its complementary signal 1702 have beenshifted slightly relative to each other, for example through the use ofoffset input 1504 of FIG. 15.

FIG. 18 is a waveform diagram illustrating a differential signal thatmay be used in conjunction with an embodiment of the present disclosure.The differential signal comprises a signal 1801 and its complementarysignal 1802. The signal 1801 and its complementary signal 1802 have beenshifted substantially relative to each other, for example through theuse of offset input 1504 of FIG. 15.

FIG. 19 is a schematic diagram illustrating an example of an input stageof an offsetable differential receiver in accordance with an embodimentof the present disclosure. A differential input signal is coupled to aninput 1901 at a gate of a first input transistor 1903 and to an input1902 at a gate of a second input transistor 1904. A source of the firstinput transistor 1903 and a source of the second input transistor 1904are coupled to a first terminal 1911 of current source 1912. A secondterminal 1913 of current source 1912 is coupled to ground.

A drain of the first input transistor 1903 is coupled to the drain oftransistor 1917, to a first terminal of resistor 1907, and to a firstoutput. A drain of the second input transistor 1904 is coupled to thedrain of transistor 1918, to a first terminal of resistor 1908, and to asecond output. The second end of the first resistor is coupled to avoltage reference 1905. The second end of the second resistor is coupledto a voltage reference 1906. The sources of transistors 1917 and 1918are coupled to a first terminal 1919 of a variable current source 1920.The second terminal 1921 of variable current source 1920 is coupled toground.

A shifting input signal 1914 is applied to the gate of transistor 1917.The shifting input signal 1914 is inverted by inverter 1915 and appliedto the gate terminal 1916 of transistor 1918.

FIG. 20 is a schematic diagram illustrating an example of an input stageof an offsetable differential receiver in accordance with an embodimentof the present disclosure. A differential input signal is coupled to aninput 2001 at a gate of a first input transistor 2003 and to an input2002 at a gate of a second input transistor 2004. A source of the firstinput transistor 2003 and a source of the second input transistor 2004are coupled to a first terminal 2011 of current source 2012. A secondterminal 2013 of current source 2012 is coupled to ground.

A drain of the first input transistor 2003 is coupled to the drain oftransistor 2017, to a first terminal of resistor 2007, and to a firstoutput. A drain of the second input transistor 2004 is coupled to thedrain of transistor 2018, to a first terminal of resistor 2008, and to asecond output. The second end of the first resistor is coupled to avoltage reference 2005. The second end of the second resistor is coupledto a voltage reference 2006. The sources of transistors 2017 and 2018are coupled to a first terminal 2019 of a current source 2020. Thesecond terminal 2021 of current source 2020 is coupled to ground. Ashifting input signal 2014 is applied to the gate of transistor 2017. Afixed DC voltage 2016 is applied to the gate terminal of transistor2018. Alternatively, a variable signal, such as a variable voltage, maybe applied to the gate terminal of transistor 2018. As one example, thevariable signal at the gate terminal of transistor 2018 may varycomplementary to the shifting input signal 2014. This would allow, forexample, a differential signal to be used to control the offset.Alternatively, the signal at the gate terminal of transistor 2018 mayvary independent of the shifting input signal 2014.

FIG. 21 is a two-dimensional graphical diagram illustrating arelationship between a waveform of a signal received at the receivecircuit and variations occurring in the interpretation of datarepresented by the signal in accordance with an embodiment of thepresent disclosure. Waveform 2101 plots a portion of the signal receivedat the receive circuit against a time axis 2102 and a voltage axis 2103.If the parameters of the system are adjusted so that sampling occurs attime 2122 with a sampling time requirement ΔT spanning range 2104 with avoltage threshold set at voltage reference 2123 with a voltage overdriverequirement of ΔV spanning range 2105, then if waveform 2101 passesthrough regions 2106-2109, repeatability of data extracted from thesignal within those regions is not guaranteed, so variations can beexpected to occur between iterations of waveform 2101 passing throughthese regions. However, if waveform 2101 passes through regions2110-2121, accurate data can be reliably extracted from waveform 2101.Thus, repeatability of the data occurs within these regions, andvariations are not detected within these regions.

By observing the variations occurring within regions 2106-2109,information representative of the locations of regions 2106-2109 can bestored. Then, the positions of time 2122 and voltage reference 2123within the plane formed by time axis 2102 and voltage axis 2103 areadjusted, and variations in extracted data are observed for samplestaken within the adjusted ranges 2104 and 2105 corresponding to theadjusted positions of time 2122 and voltage reference 2123. Informationrepresentative of locations of regions where variation is observed forthese adjusted positions is then stored. By cumulatively storing thisinformation over several iterations of this process, a representation ofwaveform 2101 can be displayed based on the information. To make therepresentation of waveform 2101 a closer approximation of the actualwaveform 2101, interpolation between the locations where variations areobserved can be performed. Such interpolation can be explicitlyperformed or allowed to occur during visualization of the representationof waveform 2101, utilizing the same visual effects that allow images ofdiscrete elements, such as dot matrix displays or bit-mapped images, toappear as though the discrete elements are merged into a larger element.

When ranges 2104 and 2105 are very small, variations may be observed inonly a single region for each iteration of positions of time 2122 andvoltage reference 2123. In that case, the accumulation of locations ofregions where variations are observed yields a thin and preciserepresentation of waveform 2101. However, when ranges 2104 and 2105 arelarger, variations may be observed over the several regions lying withinthe ranges 2104 and 2105. Thus, a thicker representation of waveform2101 results, allowing the broader area of the plane over whichvariations in extracted data occur to be observed.

FIG. 22 is a two-dimensional graphical diagram illustrating theaccumulation of locations of regions where variations are observed inaccordance with an embodiment of the present disclosure. As an example,waveform 2201 is illustrated as a triangle wave, but it should beunderstood that waveform 2201 may be of any arbitrary shape. Waveform2202 is illustrated as being sampled according to parameters that affectthe extraction of data from waveform 2201. In the example at the top ofFIG. 22, waveform 2201 is illustrated as being sampled according to avoltage reference 2202 and times 2206, 2207, and 2208, yielding points2218, 2219, and 2220, respectively, representative of the locations ofregions where variations in extracted data occur, as discussed above indetail with reference to FIG. 21. It should be understood that samplingcan also occur at any other times along waveform 2201, but that the lackof variation of data of such samples allows those samples to bedisregarded when it is desired to identify locations corresponding toregions where variations in the data occur. Also, by sampling waveform2202 according to a DC voltage reference, a waveform 2201 derived from asingle-ended signal (e.g., a signal that may be communicated over asingle conductor with reference to a reference potential, such asground) may be sampled. However, it should be noted that embodiments ofthe present disclosure may be used for waveforms other than thosederived from a single-ended signal, for example, those derived from adifferential signal, as described below with reference to FIG. 23.

In the next example of FIG. 22, waveform 2201 is illustrated as beingsampled according to a voltage reference 2203 and times 2209, 2210, and2211, yielding points 2221, 2222, and 2223, respectively, representativeof the locations of regions where variations in extracted data occur forthese adjusted parameter values. In the next example, waveform 2201 isillustrated as being sampled according to a voltage reference 2204 andtimes 2212, 2213, 2214, yielding points 2224, 2225, and 2226,respectively. In the next example, waveform 2201 is illustrated as beingsampled according to a voltage reference 2205 and times 2215, 2216, and2217, yielding points 2227, 2228, and 2229.

By accumulating points 2218-2229 and plotting them according to thevoltage references and times pertaining to their respective sampling, anapproximation 2230 of waveform 2201 can be obtained, as illustrated nearthe bottom of FIG. 22. Continuity of the approximation 2230 of waveform2201 can be obtained by interpolating between points 2218-2229, asillustrated by the dashed line of approximation 2230, or by allowingsuch continuity to be perceived when the plotted accumulation of points2218-2229 is visualized.

FIG. 23 is a two-dimensional graphical diagram illustrating theaccumulation of locations of regions where variations are observed basedon sampling pertaining to a differential signal in accordance with anembodiment of the present disclosure. A differential signal involves acomplimentary relationship between a first signal on a first conductorto a second signal on a second conductor. As with FIG. 22, an exemplarytriangle waveform is illustrated in FIG. 23, but it should be understoodthat the technique described with reference to FIG. 23 may be applied toany arbitrary waveform.

In the example illustrated at the top of FIG. 23, the first signal isrepresented by waveform 2301. The second signal is represented bywaveform 2304. In this example, waveform 2304 is used as a reference towhich waveform 2301 is compared. By sampling waveform 2301 relative towaveform 2304 at times 2310, 2313, and 2316 in accordance with thetechnique described above with reference to FIG. 21, points 2319, 2322,and 2325, respectively, are identified. By shifting waveform 2304 by anoffset 2347, as illustrated by waveform 2305, and sampling waveform 2301at times 2311, 2312, and 2317, points 2320, 2321, and 2326,respectively, are identified. By shifting waveform 2304 by an offset2348, as illustrated by waveform 2303, and sampling waveform 2301 attimes 2309, 2314, and 2315, points 2318, 2323, and 2324, respectively,are identified.

In addition to identifying points 2318-2326 by sampling waveform 2301relative to waveforms 2303-2305, another set of points 2327-2335 can beidentified by sampling waveform 2302 using waveform 2307 as a reference.In this case, the first signal is represented by waveform 2307, and thesecond signal is represented by waveform 2302. By sampling waveform 2302relative to waveform 2307 at times 2310, 2313, and 2316, points 2328,2331, and 2234, respectively, are identified. By shifting waveform 2307by an offset 2349, as illustrated by waveform 2306, and samplingwaveform 2302 at times 2309, 2314, and 2315, points 2327, 2332, and2333, respectively, are identified. By shifting waveform 2307 by anoffset 2350, as illustrated by waveform 2308, and sampling waveform 2302at times 2311, 2312, and 2317, points 2329, 2330, and 2335,respectively, are identified.

Thus, according to the times and offset shifts (or lack thereof) usedfor identifying points 2318-2335, points 2318-2335 can be plotted toyield approximations 2336 and 2337 of the waveforms corresponding to thefirst signal and the second signal, respectively, as illustrated in FIG.23. To obtain an approximation of the differential waveformcorresponding to the differential signal, the approximation 2337corresponding to the second signal is subtracted from the approximation2336 corresponding to the first signal, thereby yielding approximation2351 corresponding to the differential signal. As can be seen, points2338-2346 result from subtracting the locations of points 2327-2335 fromthe locations of points 2318-2326. Depending on the resolution of points2338-2346, the apparent continuity of approximation 2351 may be obtainedby interpolating between points 2338-2346 or by allowing such continuityto be perceived when the plotted accumulation of points 2338-2346 isvisualized.

FIGS. 24A and 24B are a flow diagram illustrating a method in accordancewith an embodiment of the present disclosure. The method begins in step2401. In step 2402, one or more parameters affecting reception of atransmit repeating pattern at a receive circuit are set. For example, atiming parameter, such as a timing parameter of a transmit circuitand/or a timing parameter of the receive circuit, and/or an amplitudeparameter, such as a voltage parameter influencing voltages of logiclevels for transmitting a transmit repeating pattern to the receivecircuit and/or a voltage parameter influencing the ability todistinguish such logic levels in the receive circuit, may be set. Instep 2403, the transmit repeating pattern is generated in a transmitcircuit. Step 2403 may include step 2404. In step 2404, a shift registermay be utilized to generate the transmit repeating pattern. Step 2404may include step 2405. In step 2405, a linear feedback shift register(LFSR) may be utilized to generate the transmit repeating pattern.

From step 2403, the method continues to step 2406. In step 2406, atransmit repeating pattern is transmitted to the receive circuit. Step2406 may include steps 2407, 2408, and/or 2409. In step 2407, thetransmit repeating pattern is transmitted as a signal referenced to aground. In step 2408, the transmit repeating pattern is transmitted as adifferential signal over a pair of conductors. In step 2409, thetransmit repeating pattern is transmitted by encoding two bits ofinformation on a single conductor simultaneously. From step 2406, themethod continues to step 2410. In step 2410, a receive repeating patternis generated in the receive circuit. From step 2410, the methodcontinues to step 2411. In step 2411, the transmit repeating pattern iscompared to the receive repeating pattern to obtain a comparison. Fromstep 2411, the method continues to step 2412. In step 2412, one or moreof the one or more parameters affecting reception of the transmitrepeating pattern at the receive circuit are adjusted. For example,parameters affecting the relative position of the transmit repeatingpattern with respect to a voltage reference or timing reference of thereceive circuit may be adjusted. Such adjustments may be made in thetransmit circuit, the receive circuit, or both. From step 2412, themethod continues to step 2413, where a decision is made as to whether ornot the process is done. If the process is done, the method continues tostep 2414, where it ends. If the process is not done, the method returnsto step 2403 and performs one or more additional iterations, allowingthe effects of adjustments performed in step 2412 to be assessed andfurther optimization to occur. Such iterations may continue to occurwithout limit. As an example, the iterations may continue with theadjustments performed over one or more ranges of the one or moreparameters to assess characteristics over the one or more ranges of theone or more parameters. For example, by adjusting a voltage and/ortiming offset parameter of the receive circuit, information can beobtained in the reiteration of step 2411. Such information can beexpressed in a coordinate system, such as a Cartesian coordinate system,and used to plot a waveform representing the transmit repeating patternas received at the receive circuit, for example, to provide in-situ“virtual oscilloscope” capability. A representation of a waveform may beconstructed based on the comparison performed in step 2411.

While a transmit clock for a transmit circuit and a receive clock for areceive circuit may operate at the same frequency (or approximately thesame frequency), the transmit clock and the receive clock may be set tooperate at frequencies that are multiple or submultiples of one another(or at frequencies that approximate such frequencies). If thefrequencies are equal, an eye diagram such as that illustrated in FIG.25 can result. The eye diagram results from the effective “folding” ofthe representation of the waveform, which results in overlapping displayof samples obtained from different cycles of the waveform. While suchoverlapping is useful in some circumstances, allowing observation ofchanges in the waveform between different cycles, such overlapping cansometimes obscure details of the representation of the waveform that aremeaningful.

It is possible to effectively “unfold” the representation of thewaveform so as to allow observation of the waveform (or a portionthereof) in detail. Such “unfolding” may be achieved by controlling thefrequency relationship between the transmit clock and the receive clock.For example, by setting the receive clock to operate slower than thetransmit clock, the representation of the waveform may be displayed ingreater detail.

Evaluation of a signaling system using repeating patterns may beperformed over one medium, for example, one conductor, with the resultsof the evaluation used to adjust one or more parameters affectingcommunication over that one medium. Alternatively, if several media, forexample, several conductors, may be characterized as providing similarperformance and are similarly affected by changes to the parameters thatrelate to them, for example, in a closely-coupled bus system, evaluationmay be performed on one medium, with the results of the evaluationapplied to the adjustment of one or more parameters affecting some orall of the several media.

As an example, an embodiment of the present disclosure may be applied toa memory device or a memory system. Media such as a data line, anaddress line, and/or a control line may be evaluated. Based on theresults of such evaluation, parameters affecting that data line, addressline, and/or control line and/or others similar to them may be adjusted.Thus, a common medium may be used for evaluation and subject to effectsof parameter adjustment. Alternatively, a medium may be subject toeffects of parameter adjustment based on evaluation involving anothermedium, namely an analysis medium. Thus, for example, a parameter may beadjusted that affects reception of a second receive data input signal,the second receive data input signal being distinct from a receive datainput signal, while that parameter may or may not affect reception ofthe receive data input signal.

As one example of an embodiment of the present disclosure, multipleevaluations may be performed for one transmit circuit coupled tomultiple receive circuits, potentially yielding multiple adjustments ofone or more parameters. Likewise, multiple evaluations may be performedfor one receive circuit coupled to multiple transmit circuit,potentially yielding multiple adjustments of one or more parameters. Asan example, in a memory system comprising multiple memory devices, amemory controller may perform separate evaluations for some or all ofthe multiple memory devices and use separate parameters to optimizecommunication with the multiple memory devices.

Evaluation of a signaling system may be performed at many differenttimes. For example, evaluation may performed during a manufacturingprocess, at system start-up, when a communication failure is detected,or during normal operation of a signaling system. Evaluation may beperformed occasionally between periods of communication of user databetween the transmit circuit and the receive circuit. As an example ofevaluation at system start-up, evaluation may be performed before thesystem is operating normally and ready to communicate user data.

While the transmit circuit and the receive circuit may be containedwithin a signaling system being evaluated, either the transmit circuitor the receive circuit may be provided externally. For example, atransmit circuit may be evaluated using an external receive circuit, ora receive circuit may be evaluated using an external transmit circuit.In one example, such external circuits may be provided in amanufacturing environment to evaluate signaling systems during theirmanufacturing process.

FIG. 25 is a waveform diagram illustrating an example of a persistentdisplay of overlapping samples of a waveform in accordance with anembodiment of the present disclosure. In this diagram, the transmitclock is operated at a slower frequency than the receive clock (in thisexample, one fourth the receive clock frequency). In this example, atransmit repeating pattern of 1111 0000 1111 0000 is used. As can beseen, multiple cycles overlap, resulting in edges 2501 and 2502, whichrepresent rising edges for some cycles and falling edges for othercycles. As can be seen from levels 2503 and 2504, for some cycles, notransition occurred at the time corresponding to edge 2501. As can beseen from levels 2506 and 2507, for some cycles, no transition occurredat the time corresponding to edge 2502. While such information is usefulunder some circumstances, it can obscure desired details under othercircumstances.

FIG. 26 is a waveform diagram illustrating an example of a display ofsamples of a waveform in accordance with an embodiment of the presentdisclosure. In this diagram, the transmit clock is operated at a higherfrequency than the receive clock (in this example, four times thereceive clock frequency). In this example, a transmit repeating patternof 1111 0000 1111 0000 is used. As can be seen, greater detail of thewaveform can be observed, as rising edge 2601 is clearly visible, notoverlapping with a falling edge, and falling edge 2603 is clearlyvisible, not overlapping with a rising edge. High level 2602 is clearlyvisible, not overlapping with cycles lacking rising edge 2601.Consequently, ringing effect 2604 can be observed in detail Likewise,ringing effect 2605 can be observed in detail.

A display of samples of a waveform such as that illustrated in FIG. 26is useful for observing the step response of a signaling system. Thestep response is a characterization of how a system is affected by asignal having a rapid transition from one level to another. In theexample of FIG. 26, a rapid transition from a low level to a high level2602 occurs at rising edge 2601. The ringing effect 2604 is associatedwith the step response of the system. Thus, to observe a step responseof a system, a signal having a rapid transition from one level toanother may be applied to the system and the resulting system behaviorobserved. Embodiments of the present disclosure may be used to observesuch system behavior. Awareness of the step response of a system can bereadily used for system optimization.

FIG. 27 is a waveform diagram illustrating an example of a display ofsamples of a waveform in accordance with an embodiment of the presentdisclosure. This example may also be applied to determination of a stepresponse. In this diagram, the transmit clock is operated at a higherfrequency than the receive clock (in this example, four times thereceive clock frequency). In this example, a simplified transmitrepeating pattern of 1111 1111 0000 000 is used. As can be seen,reducing the receive clock frequency can be used to allow display of thewaveform over multiple symbol times. In this example, rising edge 2701leads to high level 2702, allowing detailed observation of ringingeffect 2703. Also, falling edge 2704 leads to low level 2705, allowingdetailed observation of ringing effect 2706. While rising edge 2701overlaps with falling edge 2704, the greatly expanded detail with whichthe waveform is displayed due to combinations of a simplified patternand clocking at a lower frequency avoids the problems of the rising edge2701 and the falling edge 2704 obscuring one another.

In accordance with an embodiment of the present disclosure, an iterativeprocess may be applied to adjust system parameters so as to minimize theringing effects 2703 and 2706 illustrated in FIG. 27. By reducing thefrequency of the receive circuit clock relative to the transmit circuitclock, multiple symbols may be observed. Ringing patterns 2703 and 2706may be observed regardless of whether they are one symbol or multiplesymbols away from a causative transition. For example, in a transmissionline environment, a transition may cause a disturbance, such as theringing pattern 2703 or 2706, that propagates along a transmission lineuntil it is reflected at some point along the transmission line and isobserved at some later time relative to the time at which the causativetransition occurred. By properly adjusting the timing relationshipsaffecting the portion of the signal being observed, such disturbancescan be observed regardless of their temporal position relative to thecausative transition.

FIG. 28 is a waveform diagram illustrating an example of a persistentdisplay of overlapping samples of a waveform in accordance with anembodiment of the present disclosure. One embodiment of the presentdisclosure may be used to provide information similar to that which istraditionally obtainable through the use of an oscilloscope. In the testmode, comparison can be made between a pattern generated in the transmitcircuit and a pattern generated in the receive circuit. This testing canbe reiterated for different receiver timing and overdrive conditionswhile the signal representing the pattern is being compared. When thetime and overdrive condition of the receiver is varied to the regionwhere the pattern comparisons yield inconsistent results, the failingregion corresponds to a metastable region of the receiver. When thesemetastable regions are plotted, they outline the signal waveform that isbeing received at the receiver, including the signal uncertainty(jitter) and the receiver timing and overdrive deadband requirement.

Hence, the outline of these metastable regions represent the signal asseen by the receiver with its own receiving characteristics. With thiscapability of visualizing the signal, various effects on the signal canbe checked out. For example, an output current level, crosstalk,attenuation, etc. In case of unwanted signal integrity behaviors,different compensation techniques can be used to reduce or eliminatethose behaviors. The term “metastable” as used herein refers to a regionwherein the receiver is unable to reliably identify a level of anincoming signal. Thus, for the same level of the incoming signal, thereceiver will, on different occasions, identify that level to bedifferent levels. Thus, for repetitions of a given incoming signal, thereceiver will not provide a repeatable output within the metastableregion. Thus, the metastable region may also be referred to as a regionof unrepeatability. Consequently, to identify such a region, thetransmit circuit and the receive circuit may be configured to operateusing patterns having lengths or periods that bear a multiple andsubmultiple relationship to each other, and any lack of repeatability ofthe receiver may be observed. As an example, the length or period of atransmit repeating pattern may be a multiple of the length or period ofa receive repeating pattern, and the length or period of the receiverepeating pattern may be a submultiple of the length or period of thetransmit repeating pattern. As another example, the length or period ofa receive repeating pattern may be a multiple of the length or period ofa transmit repeating pattern, and the length or period of the transmitrepeating pattern may be a submultiple of the length or period of thereceive repeating pattern. A transmit clock rate of a transmit repeatingpattern and a receive clock rate of a receive repeating pattern may beara multiple and submultiple relationship to one another. One subset ofall possible multiple and submultiple relationships is a one-to-onerelationship.

The example of FIG. 28 illustrates samples of waveforms exhibiting theeffects of crosstalk induced by other nearby conductors, as can be seen,for example at locations 2801, 2802, 2805, and 2804.

FIG. 29 is a waveform diagram illustrating an example of a persistentdisplay of overlapping samples of a waveform in accordance with anembodiment of the present disclosure. The example of FIG. 29 showsresults obtained using “step response” types of waveforms. Locations2901 and 2902 illustrate effects of an impedance discontinuity in themedium between the transmit circuit and the receive circuit.

FIG. 30 is a waveform diagram illustrating an example of a persistentdisplay of overlapping samples of a waveform in accordance with anembodiment of the present disclosure. The example of FIG. 30 illustrateda 4-level pulse amplitude modulation (4-PAM) generated using patternsrepresenting a stair-step-type signal. Differences between the risingedges 3001, 3002, 3003, 3004, 3005, and 3006 and their respectivefalling edges 3007, 3008, 3009, 3010, 3011, and 3012 are observable.

Thus, in view of FIGS. 25-30, it can be seen that an embodiment of thepresent disclosure provides a powerful tool for in-situ characterizationand optimization of signaling systems. Characteristics that could not bedetermined using traditional test equipment are readily ascertainable inaccordance with an embodiment of the present disclosure.

FIG. 31 is a block diagram illustrating a system in accordance with anembodiment of the present disclosure. The system comprises transmitcircuit 3101, medium 3102, and receive circuit 3103. Transmit circuit3101 comprises a shift register 3105 having a parallel load input 3104to load data from register 3121. A transmit circuit load signal coupledto an input of register 3121 at node 3122 is used to control the loadingof data. A feedback loop 3106 couples a serial data output of shiftregister 3105 to a serial data input of shift register 3105. The serialdata output of shift register 3105 is also coupled to an input oftransmitter 3115. An output of transmitter 3115 is coupled to medium3102.

Receiver circuit 3103 comprises receiver 3116, multiplexer 3119, shiftregister 3108, and XOR gate 3113. Medium 3102 is coupled to an input ofreceiver 3116. A receive circuit timing signal is coupled to an input ofreceiver 3116 at node 3117. A voltage reference signal is coupled to aninput of receiver 3116 at node 3118. An output of receiver 3116 iscoupled to an input of multiplexer 3119 and to an input of XOR gate3113. A fill pipe signal is coupled to a selection input of multiplexer3119 at node 3120. An output of multiplexer 3119 is coupled to a serialdata input of shift register 3108. A serial data output of shiftregister 3108 is coupled to an input of multiplexer 3119 and to an inputof XOR gate 3113 via line 3111. An output of XOR gate 3113 provides anerror output at node 3114.

FIG. 32 is a flow diagram illustrating a method in accordance with anembodiment of the present disclosure. The method begins in step 3201 andcontinues to step 3202. In step 3202, mapping of a system waveform isperformed, for example, according to some or all of steps 2402 through2412 of FIG. 24. From step 3202, the method proceeds to step 3203. Instep 3203, a system performance characteristic is evaluated based oninformation obtained in step 3202. For example, a system performancecharacteristic, such as a voltage margin, a timing margin, a value of avoltage level, or timing of an edge, may be evaluated. In step 3204, oneor more system parameters are adjusted. These system parameters mayinclude, for example, an output current, a crosstalk cancellationcoefficient, a self-equalization coefficient, a receive circuit timingsignal, and a voltage reference.

From step 3204, the method continues to step 3205. In step 3205, mappingof a system waveform is performed, for example, in a manner as describedin relation to step 3202. From step 3205, the method continues to step3206. In step 3206, a system performance characteristic is evaluated,for example, in a manner as described in relation to step 3203. In step3207, the results of the evaluation of step 3203 and the evaluation ofstep 3206 are compared. In step 3208, a determination is made based onthe comparison of 3207. If the results of the evaluation of step 3206are better (e.g., closer to a desired performance level of a systemperformance characteristic) than the results of the evaluation of step3203, the method returns to step 3204 for further adjustment of one ormore system parameters using the same direction or sign of adjustment.If, however, the results of the evaluation of step 3206 are not betterthan the results of the evaluation of step 3203, the method continues tostep 3209. In step 3209, the direction or sign of the system parameteradjustment to be performed is changed. From step 3209, the methodreturns to step 3204 for further adjustment of one or more systemparameters in accordance with the change of direction or sign providedin step 3209.

Iterations of this method may continue as long as desired, for example,until a desired performance level of a system performance characteristicis obtained. As another example, iterations may continue until anoptimal level of a system performance characteristic is reached.Multiple iterations of the method can be performed to allow adjustmentof each system performance characteristic to its optimal level. Forexample, one system performance characteristic can be optimized, then asecond system performance characteristic can be optimized, therebyproviding sequential optimization of multiple system performancecharacteristics. Once a desired or optimal performance level of thesystem is obtained, the process illustrated in FIG. 32 may end.

The method of FIG. 32 may be performed until the system reaches agenerally steady state condition. Such steady state conditions may bedefined at a plurality of levels. For example, at a broader level, thesteady state condition may be more approximate, while, at a narrowerlevel, the steady state condition may be more precise. As an example, asteady state condition may be identified when step 3209 occurs,especially if it occurs several times over a relatively small range ofadjustment of the relevant system parameter. Adjustment back and forthwithin a limited range may be observed as “dithering” and may be used toindicate completion of the method of FIG. 32. If hysteresis occursduring adjustment, such that no specific value for the system parameteris identified for the steady state condition, a value can beinterpolated within the range of adjustment observed. For example, avalue in the middle of the range may be selected. When the method ofFIG. 32 is completed for one system parameter, it may be repeated for adifferent system parameter. Alternatively, multiple system parametersmay be adjusted simultaneously.

FIG. 33 is a waveform diagram illustrating optimization of crosstalkcancellation resulting from iterative application of the methodillustrated in FIG. 32 in accordance with an embodiment of the presentdisclosure. Waveforms 3301, 3302, 3303, 3304, and 3305, representing anon-transitioning portion of a signal subject to iterative applicationof the method described in relation to FIG. 32, are illustrated inrelation to a horizontal axis 3313 and a vertical axis 3314. Thehorizontal axis 3313 may represent, for example, time, while thevertical axis 3314 may represent, for example, an amplitude, such as avoltage.

Waveform 3306, representing a transitioning portion of an adjacentsignal, is illustrated in relation to the horizontal axis 3313 and avertical axis 3315, which may represent, for example, an amplitude, suchas a voltage. Waveforms 3301, 3302, 3303, 3304, and 3305 exhibitdisturbances 3308, 3309, 3310, 3311, and 3312, respectively, whichresult from the influence of the sharply rising edge 3307 of waveform3306, which causes a crosstalk phenomenon.

With a first set of values of system parameters, the disturbance 3308occurs when waveform 3301 is influenced by waveform 3306. By adjustingsystem parameters, the amplitude of the disturbance can be reduced.Thus, with a second set of values of system parameters, the disturbance3309 of waveform 3302 is of a lesser amplitude than disturbance 3308 ofwaveform 3301. Yet, the crosstalk phenomenon is still undercompensated.A third set of values of system parameters yields a further reducedamplitude of disturbance 3310 of waveform 3303. A fourth set of valuesof system parameters results in waveform 3304 being almost immune to thecrosstalk phenomenon, exhibiting only disturbance 3311 of very slightamplitude. A fifth set of values of system parameters overcompensatesfor the influence of waveform 3306, resulting in disturbance 3312 of anopposite polarity affecting waveform 3305.

Therefore, using by detecting undercompensation and overcompensation,the iterative application of the method described in reference to FIG.32 can be used to find optimal values for system parameters, forexample, the system parameters that result in waveform 3304. If desired,additional iterations could be performed using sets of values of systemparameters between those of waveforms 3303 and 3305 to more finelyadjust the optimal values.

As one skilled in the art can readily appreciate, there are myriadsystem parameters that can be optimized in accordance with an embodimentof the present disclosure. Some examples of parameters that can beadjusted in this manner include equalization coefficients, crosstalkcancellation coefficients, output drive levels, termination settings,transmit and receive clock offsets, input receiver windows, as well asmany others. A termination setting is a parameter that affects atermination impedance of a transmission line. A transmit clock offset isa parameter that affects the temporal position of a clock signal in atransmit circuit or the temporal position of a signal transmitted by atransmit circuit. A receive clock offset is a parameter that affects thetemporal position of a clock signal in a receive circuit, the temporalposition of a signal used by the receive circuit for receiving atransmitted signal, or the temporal position of a sampling time duringwhich a transmitted signal is sampled by the receive circuit. An inputreceiver window is a parameter affecting the differentiation ofdifferent logic levels in the receive circuit. An output drive level isa parameter affecting the representation of different logic levels atthe transmit circuit. A crosstalk cancellation coefficient is aparameter affecting immunity to crosstalk caused by signals on otherconductors. An equalization coefficient is a parameter that may beadjusted to effect equalization adjustment.

FIG. 2 is a logic diagram illustrating a prior art transmit circuit.Node 201 is coupled to a serial data input of a shift registercomprising flip-flops 215, 217, 219, and 221. A load signal forperforming a parallel data load of the shift register is provided to theshift register at node 203. A transmit clock signal is provided to aclock input of the shift register at node 205. Nodes 207, 209, 211, and213 are coupled to parallel load data inputs of flip-flops 215, 217,219, and 221, respectively. The serial data output of the shift registerat the output of flip-flop 221 is coupled to node 223, which is coupledto an input of multiplexer 226.

Node 202 is coupled to a serial data input of a shift registercomprising flip-flops 216, 218, 220, and 222. A load signal forperforming a parallel data load of the shift register is provided to theshift register at node 204. A transmit clock signal is provided to aclock input of the shift register at node 206. Nodes 208, 210, 212, and214 are coupled to parallel load data inputs of flip-flops 216, 218,220, and 222, respectively. The serial data output of the shift registerat the output of flip-flop 222 is coupled to node 224, which is coupledto an input of multiplexer 226.

A transmit clock signal is provided to a input of multiplexer 226 atnode 225. The output of multiplexer 226 is coupled to node 230, which iscoupled to an input of output driver 227. An output of output driver 227is coupled to node 228, which is coupled to pad 229.

FIG. 3 is a logic diagram illustrating a prior art receive circuit. Pad301 is coupled to node 302, which is coupled to an input of evenreceiver 305 and to an input of odd receiver 306. A receive clock signalis provided to an input of even receiver 305 at node 303 and to an inputof odd receiver 306 at node 304. An output of even receiver 305 at node307 is coupled to an input of a shift register comprising flip-flops313, 315, 317, and 319. A receive clock is provided to a clock input ofthe shift register at node 309. An unload signal for providing paralleldata outputs from the shift register is applied to the shift registervia node 311. Parallel data are provided at parallel data outputs 321,323, 325, and 327 of flip-flops 313, 315, 317, and 319, respectively.

An output of odd receiver 306 at node 308 is coupled to an input of ashift register comprising flip-flops 314, 316, 318, and 320. A receiveclock is provided to a clock input of the shift register at node 310. Anunload signal for providing parallel data outputs from the shiftregister is applied to the shift register via node 312. Parallel dataare provided at parallel data outputs 322, 324, 326, and 328 offlip-flops 314, 316, 318, and 320, respectively.

FIG. 4 is a logic diagram illustrating a transmit circuit capable ofoperating in a PRBS test mode in accordance with an embodiment of thepresent disclosure. Node 401 is coupled to a serial data input of ashift register comprising flip-flops 415, 417, 419, and 421. A loadsignal for performing a parallel data load of the shift register isprovided to the shift register at node 403. A transmit clock signal isprovided to a clock input of the shift register at node 405. Nodes 407,409, 411, and 413 are coupled to parallel load data inputs of flip-flops415, 417, 419, and 421, respectively.

The serial data output of the shift register at the output of flip-flop421 is coupled to node 423, which is coupled to an input of multiplexer426. Node 423 is also coupled to an input of XOR gate 433. A signal atnode 431 taken from the data output of flip-flop 417 is coupled to aninput of XOR gate 433. The output of XOR gate 433 is coupled to an inputof multiplexer 446 via node 435. A fixed logic zero signal is coupled toan input of multiplexer 446 at node 442. A PRBS test signal is coupledto a selection input of multiplexer 446 via node 444. The output ofmultiplexer 446 is coupled to node 402.

Node 402 is coupled to a serial data input of a shift registercomprising flip-flops 416, 418, 420, and 422. A load signal forperforming a parallel data load of the shift register is provided to theshift register at node 404. A transmit clock signal is provided to aclock input of the shift register at node 406. Nodes 408, 410, 412, and414 are coupled to parallel load data inputs of shift registers 416,418, 420, and 422, respectively.

Node 424 is taken from a serial data output of the shift register at theoutput of flip-flop 422 and is coupled to an input of multiplexer 426and to an input of XOR gate 434. A signal at node 432 taken from thedata output of flip-flip 418 is coupled to an input of XOR gate 434. Theoutput of XOR gate 434 appears at node 436, which is coupled to an inputof flip-flop 438.

A transmit clock signal is provided to a clock input of flip-flop 438via node 437. An output of flip-flop 438 is coupled to an input ofmultiplexer 445 via node 439. A fixed logic zero input is coupled to aninput of multiplexer 445 at node 441. A PRBS test signal is coupled to aselection input of multiplexer 445 via node 443. An output ofmultiplexer 445 is coupled to node 401.

A transmit clock signal is provided to multiplexer 426 via node 425. Theoutput of multiplexer of 426 is coupled via node 430 to an input ofoutput driver 427. Output driver 427 provides an output at node 428,which is coupled to pad 429.

FIG. 5 is a logic diagram illustrating a receive circuit capable ofoperating in a PRBS test mode in accordance with an embodiment of thepresent disclosure. Pad 501 is coupled to node 502, which is coupled toeven receiver 505 and odd receiver 506. A receive clock signal isprovided to even receiver 505 at node 503 and to odd receiver 506 atnode 504.

An output of even receiver 505 is coupled to an input of multiplexer 543and to an input of XOR gate 547 via node 507. The output of multiplexer543 is coupled to an input of XOR gate 547 and to a serial data input ofa shift register comprising flip-flops 513, 515, 517, and 519 via node545. An output of XOR gate 547 is coupled to an input of OR gate 551 atnode 549.

A receive clock signal is provided to a clock input of the shiftregister via node 509. An unload signal for providing parallel dataoutputs from the shift register is applied to the shift register vianode 511. Parallel data are provided at parallel data outputs 521, 523,525, and 527 of flip-flops 513, 515, 517, and 519, respectively.

An output of flip-flop 519 is coupled to an input of XOR gate 533 vianode 529. An output of flip-flop 515 is coupled to an input of XOR gate533 via node 531. An output of XOR gate 533 is coupled to an input ofmultiplexer 544 via node 535. A PRBS test signal is applied to aselection input 542 of multiplexer 544.

An output of odd receiver 506 is coupled to an input of multiplexer 544and to an input of XOR gate 548 via node 508. The output of multiplexer544 is coupled to an input of XOR gate 548 and to the serial data inputof a shift register comprising flip-flops 514, 516, 518, and 520 vianode 546. An output of XOR gate 548 is coupled to an input of OR gate551 at node 550.

A receive clock signal is provided to a clock input of the shiftregister via node 510. An unload signal for providing parallel dataoutputs from the shift register is applied to the shift register vianode 512. Parallel data are provided at parallel data outputs 522, 524,526, and 528 of flip-flops 514, 516, 518, and 520, respectively.

An output of flip-flop 520 is coupled to an input of XOR gate 534 vianode 530. An output of flip-flop 516 is coupled to an input of XOR gate534 via node 532. An output of XOR gate 534 is coupled to an input offlip-flop 538 via node 536. A receive clock signal is applied to a clockinput 537 of flip-flop 538. An output of flip-flop 538 is coupled to aninput of multiplexer 543 via node 539. A PRBS test signal is applied toa selection input 541 of multiplexer 543.

An output of OR gate 551 at node 552 is coupled to an input of flip-flop555. A receive clock signal is provided to a clock input of flip-flop555 at node 553. A PRBS test signal is applied to an input of flip-flop555 at node 554. An error flag output of flip-flop 555 is provided atnode 556.

FIG. 6 is a logic diagram illustrating a transmit circuit capable ofoperating in a PRBS test mode and a roll test mode in accordance with anembodiment of the present disclosure. Node 601 is coupled to a serialdata input of a shift register comprising flip-flops 615, 617, 619, and621. A load signal for performing a parallel data load of the shiftregister is provided to the shift register at node 603. A transmit clocksignal is provided to a clock input of the shift register at node 605.Nodes 607, 609, 611, and 613 are coupled to parallel load data inputs offlip-flops 615, 617, 619, and 621, respectively.

The serial data output of the shift register at the output of flip-flop621 is coupled to node 623, which is coupled to an input of multiplexer626. Node 623 is also coupled to an input of XOR gate 633 and to aninput of multiplexer 645. A signal at node 631 taken from the dataoutput of flip-flop 617 is coupled to an input of XOR gate 633. Theoutput of XOR gate 633 is coupled to an input of multiplexer 646 vianode 635. A fixed logic zero signal is coupled to an input ofmultiplexer 646 at node 642. A PRBS test signal is coupled to aselection input of multiplexer 646 via node 644. A roll test signal iscoupled to an input of multiplexer 646 via node 648. The output ofmultiplexer 646 is coupled to node 602.

Node 602 is coupled to a serial data input of a shift registercomprising flip-flops 616, 618, 620, and 622. A load signal forperforming a parallel data load of the shift register is provided to theshift register at node 604. A transmit clock signal is provided to aclock input of the shift register at node 606. Nodes 608, 610, 612, and614 are coupled to parallel load data inputs of shift registers 616,618, 620, and 622, respectively.

Node 624 is taken from a serial data output of the shift register at theoutput of flip-flop 622 and is coupled to an input of multiplexer 626,an input of XOR gate 634, and an input of multiplexer 646. A signal atnode 632 taken from the data output of flip-flip 618 is coupled to aninput of XOR gate 634. The output of XOR gate 634 appears at node 636,which is coupled to an input of flip-flop 638.

A transmit clock signal is provided to a clock input of flip-flop 638via node 637. An output of flip-flop 638 is coupled to an input ofmultiplexer 645 via node 639. A fixed logic zero input is coupled to aninput of multiplexer 645 at node 641. A PRBS test signal is coupled to aselection input of multiplexer 645 via node 643. A roll test signal iscoupled to a selection input of multiplexer 645 via node 647. An outputof multiplexer 645 is coupled to node 601.

A transmit clock signal is provided to multiplexer 626 via node 625. Theoutput of multiplexer of 626 is coupled via node 630 to an input ofoutput driver 627. Output driver 627 provides an output at node 628,which is coupled to pad 629.

FIG. 7 is a logic diagram illustrating a receive circuit capable ofoperating in a PRBS test mode and a roll test mode in accordance with anembodiment of the present disclosure. Pad 701 is coupled to node 702,which is coupled to even receiver 705 and odd receiver 706. A receiveclock signal is provided to even receiver 705 at node 703 and to oddreceiver 706 at node 704.

An output of even receiver 705 is coupled to an input of multiplexer 743and to an input of XOR gate 747 via node 707. The output of multiplexer743 is coupled to an input of XOR gate 747 and to a serial data input ofa shift register comprising flip-flops 713, 715, 717, and 719 via node745. An output of XOR gate 747 is coupled to an input of OR gate 751 atnode 749.

A receive clock signal is provided to a clock input of the shiftregister via node 709. An unload signal for providing parallel dataoutputs from the shift register is applied to the shift register vianode 711. Parallel data are provided at parallel data outputs 721, 723,725, and 727 of flip-flops 713, 715, 717, and 719, respectively.

An output of flip-flop 719 is coupled to an input of XOR gate 733 and toan input of multiplexer 743 via node 729. An output of flip-flop 715 iscoupled to an input of XOR gate 733 via node 731. An output of XOR gate733 is coupled to an input of multiplexer 744 via node 735. A PRBS testsignal is applied to a selection input 742 of multiplexer 744. A rolltest signal is applied to a selection input 788 of multiplexer 744.

An output of odd receiver 706 is coupled to an input of multiplexer 744and to an input of XOR gate 748 via node 708. The output of multiplexer744 is coupled to an input of XOR gate 748 and to the serial data inputof a shift register comprising flip-flops 714, 716, 718, and 720 vianode 746. An output of XOR gate 748 is coupled to an input of OR gate751 at node 750.

A receive clock signal is provided to a clock input of the shiftregister via node 710. An unload signal for providing parallel dataoutputs from the shift register is applied to the shift register vianode 712. Parallel data are provided at parallel data outputs 722, 724,726, and 728 of flip-flops 714, 716, 718, and 720, respectively.

An output of flip-flop 720 is coupled to an input of XOR gate 734 and toan input of multiplexer 744 via node 730. An output of flip-flop 716 iscoupled to an input of XOR gate 734 via node 732. An output of XOR gate734 is coupled to an input of flip-flop 738 via node 736. A receiveclock signal is applied to a clock input 737 of flip-flop 738. An outputof flip-flop 738 is coupled to an input of multiplexer 743 via node 739.A PRBS test signal is applied to a selection input 741 of multiplexer743. A roll test signal is applied to a selection input 787 ofmultiplexer 743.

An output of OR gate 751 at node 752 is coupled to an input of flip-flop755. A receive clock signal is provided to a clock input of flip-flop755 at node 753. A PRBS test or roll test signal is applied to an inputof flip-flop 755 at node 754. An error flag output of flip-flop 755 isprovided at node 756.

FIG. 8 is a logic diagram illustrating a prior art quad signaling leveltransmit circuit. A fixed logic zero signal is coupled at node 801 to aninput of a shift register comprising flip-flops 815, 817, 819, and 821.A load signal for performing a parallel data load of the shift registeris provided to the shift register at node 803. A transmit clock signalis provided to a clock input of the shift register at node 805. Nodes807, 809, 811, and 813 are coupled to parallel load data inputs offlip-flops 815, 817, 819, and 821, respectively. A serial data output ofthe shift register at the output of flip-flop 821 is coupled to an inputof multiplexer 826 at node 823.

A fixed logic zero signal is coupled at node 802 to an input of a shiftregister comprising flip-flops 816, 818, 820, and 822. A load signal forperforming a parallel data load of the shift register is provided to theshift register at node 804. A transmit clock signal is provided to aclock input of the shift register at node 806. Nodes 808, 810, 812, and814 are coupled to parallel load data inputs of flip-flops 816, 818,820, and 822, respectively. A serial data output of the shift registerat the output of flip-flop 822 is coupled to an input of multiplexer 826at node 824.

A fixed logic zero signal is coupled at node 851 to an input of a shiftregister comprising flip-flops 865, 867, 869, and 871. A load signal forperforming a parallel data load of the shift register is provided to theshift register at node 853. A transmit clock signal is provided to aclock input of the shift register at node 855. Nodes 857, 859, 861, and863 are coupled to parallel load data inputs of flip-flops 865, 867,869, and 871, respectively. A serial data output of the shift registerat the output of flip-flop 871 is coupled to an input of multiplexer 876at node 873.

A fixed logic zero signal is coupled at node 852 to an input of a shiftregister comprising flip-flops 866, 868, 870, and 872. A load signal forperforming a parallel data load of the shift register is provided to theshift register at node 854. A transmit clock signal is provided to aclock input of the shift register at node 856. Nodes 858, 860, 862, and864 are coupled to parallel load data inputs of flip-flops 866, 868,870, and 872, respectively. A serial data output of the shift registerat the output of flip-flop 872 is coupled to an input of multiplexer 876at node 874.

A transmit clock signal is provided to multiplexer 826 via node 825. Theoutput of multiplexer of 826 is coupled via node 830 to an input ofoutput driver 827. A transmit clock signal is coupled to an input ofmultiplexer of 876 via node 875. The output of multiplexer 876 iscoupled to output driver 827 via node 880. Output driver 827 provides anoutput at node 828, which is coupled to pad 829.

FIG. 9 is a logic diagram illustrating a quad signaling level transmitcircuit capable of operating in a PRBS test mode in accordance with anembodiment of the present disclosure. Node 901 is coupled to a serialdata input of a shift register comprising flip-flops 915, 917, 919, and921. A load signal for performing a parallel data load of the shiftregister is provided to the shift register at node 903. A transmit clocksignal is provided to a clock input of the shift register at node 905.Nodes 907, 909, 911, and 913 are coupled to parallel load data inputs offlip-flops 915, 917, 919, and 921, respectively.

The serial data output of the shift register at the output of flip-flop921 is coupled to node 923, which is coupled to an input of multiplexer926. Node 923 is also coupled to an input of XOR gate 933. A signal atnode 931 taken from the data output of flip-flop 915 is coupled to aninput of XOR gate 933. The output of XOR gate 933 is coupled to an inputof multiplexer 946 via node 935. A fixed logic zero signal is coupled toan input of multiplexer 946 at node 942. A PRBS test signal is coupledto a selection input of multiplexer 946 via node 944. The output ofmultiplexer 946 is coupled to node 902.

Node 902 is coupled to a serial data input of a shift registercomprising flip-flops 916, 918, 920, and 922. A load signal forperforming a parallel data load of the shift register is provided to theshift register at node 904. A transmit clock signal is provided to aclock input of the shift register at node 906. Nodes 908, 910, 912, and914 are coupled to parallel load data inputs of shift registers 916,918, 920, and 922, respectively.

Node 924 is taken from a serial data output of the shift register at theoutput of flip-flop 922 and is coupled to an input of multiplexer 926and to an input of XOR gate 934. A signal at node 932 taken from thedata output of flip-flip 916 is coupled to an input of XOR gate 934. Theoutput of XOR gate 934 appears at node 936, which is coupled to an inputof multiplexer 995. A fixed logic zero signal is coupled to an input ofmultiplexer 995 via node 991. A PRBS test signal is coupled to aselection input of multiplexer 995 via node 993. An output ofmultiplexer 995 is coupled to node 951.

Node 951 is coupled to a serial data input of a shift registercomprising flip-flops 965, 967, 969, and 971. A load signal forperforming a parallel data load of the shift register is provided to theshift register at node 953. A transmit clock signal is provided to aclock input of the shift register at node 955. Nodes 957, 959, 961, and963 are coupled to parallel load data inputs of flip-flops 965, 967,969, and 971, respectively.

Node 973 is taken from the serial data output of the shift register atthe output of flip-flop 971 and is coupled to an input of multiplexer976 and to an input of XOR gate 983. Node 981 provides a signal takenfrom the data output of flip-flop of 965 and provides it to an input ofXOR gate 983. The output of XOR gate 983 appears at node 985 and iscoupled to an input of multiplexer of 996. A fixed logic zero signal iscoupled to an input of multiplexer 996 via input 992. A PRBS test signalis coupled to a selection input of multiplexer 996 via node 994. Theoutput of multiplexer 996 is coupled to node 952.

Node 952 is coupled to a serial data input of a shift registercomprising flip-flops 966, 968, 970, and 972. A load signal forperforming a parallel data load of the shift register is provided to theshift register at node 954. A transmit clock signal is provided to aclock input of the shift register at node 956. Nodes 958, 960, 962, and964 are coupled to parallel load data inputs of flip-flops 966, 968,970, and 972, respectively.

Node 974 provides a signal taken from the serial data output of theshift register at the output of flip-flop 972 to an input of multiplexer976 and to an input of XOR gate 984. Node 982 is taken from a dataoutput of flip-flop 966 and coupled to an input of XOR gate 984. XORgate 984 provides an output at node 986, which is coupled to an input offlip-flop 938.

A transmit clock signal is provided to a clock input of flip-flop 938via node 937. A PRBS test input signal is provided to an inverted inputof flip-flop 938 via node 949.

An output of flip-flop 938 is coupled to an input of multiplexer 945 vianode 939. A fixed logic zero input is coupled to an input of multiplexer945 at node 941. A PRBS test signal is coupled to a selection input ofmultiplexer 945 via node 943. An output of multiplexer 945 is coupled tonode 901.

A transmit clock signal is provided to multiplexer 926 via node 925. Theoutput of multiplexer of 926 is coupled via node 930 to an input ofoutput driver 927. A transmit clock signal is coupled to an input ofmultiplexer of 976 via node 975. The output of multiplexer 976 iscoupled to output driver 927 via node 980. Output driver 927 provides anoutput at node 928, which is coupled to pad 929.

FIG. 10 is a logic diagram illustrating a quad signaling level transmitcircuit capable of operating in a PRBS mode and a roll test mode inaccordance with an embodiment of the present disclosure. Node 1001 iscoupled to a serial data input of a shift register comprising flip-flops1015, 1017, 1019, and 1021. A load signal for performing a parallel dataload of the shift register is provided to the shift register at node1003. A transmit clock signal is provided to a clock input of the shiftregister at node 1005. Nodes 1007, 1009, 1011, and 1013 are coupled toparallel load data inputs of flip-flops 1015, 1017, 1019, and 1021,respectively.

The serial data output of the shift register at the output of flip-flop1021 is coupled to node 1023, which is coupled to an input ofmultiplexer 1026. Node 1023 is also coupled to an input of XOR gate 1033and to an input of multiplexer 1045. A signal at node 1031 taken fromthe data output of flip-flop 1015 is coupled to an input of XOR gate1033. The output of XOR gate 1033 is coupled to an input of multiplexer1046 via node 1035. A fixed logic zero signal is coupled to an input ofmultiplexer 1046 at node 1042. A PRBS test signal is coupled to aselection input of multiplexer 1046 via node 1044. A roll test signal iscoupled to an input of multiplexer 1046 via node 1048. The output ofmultiplexer 1046 is coupled to node 1002.

Node 1002 is coupled to a serial data input of a shift registercomprising flip-flops 1016, 1018, 1020, and 1022. A load signal forperforming a parallel data load of the shift register is provided to theshift register at node 1004. A transmit clock signal is provided to aclock input of the shift register at node 1006. Nodes 1008, 1010, 1012,and 1014 are coupled to parallel load data inputs of shift registers1016, 1018, 1020, and 1022, respectively.

Node 1024 is taken from a serial data output of the shift register atthe output of flip-flop 1022 and is coupled to an input of multiplexer1026, an input of XOR gate 1034, and an input of multiplexer 1046. Asignal at node 1032 taken from the data output of flip-flip 1016 iscoupled to an input of XOR gate 1034. The output of XOR gate 1034appears at node 1036, which is coupled to an input of multiplexer 1095.A fixed logic zero signal is coupled to an input of multiplexer 1095 vianode 1091. A PRBS test signal is coupled to a selection input ofmultiplexer 1095 via node 1093. A roll test signal is coupled to aselection input of multiplexer of 1095 via node 1097. An output ofmultiplexer 1095 is coupled to node 1051.

Node 1051 is coupled to a serial data input of a shift registercomprising flip-flops 1065, 1067, 1069, and 1071. A load signal forperforming a parallel data load of the shift register is provided to theshift register at node 1053. A transmit clock signal is provided to aclock input of the shift register at node 1055. Nodes 1057, 1059, 1061,and 1063 are coupled to parallel load data inputs of flip-flops 1065,1067, 1069, and 1071, respectively.

Node 1073 is taken from the serial data output of the shift register atthe output of flip-flop 1071 and is coupled to an input of multiplexer1076, to an input of XOR gate 1083, and to an input of multiplexer 1095.Node 1081 provides a signal taken from the data output of flip-flop of1065 and provides it to an input of XOR gate 1083. The output of XORgate 1083 appears at node 1085 and is coupled to an input of multiplexerof 1096. A fixed logic zero signal is coupled to an input of multiplexer1096 via input 1092. A PRBS test signal is coupled to a selection inputof multiplexer 1096 via node 1094. A roll test signal is coupled to aselection input of multiplexer 1096 via node 1098. The output ofmultiplexer 1096 is coupled to node 1052.

Node 1052 is coupled to a serial data input of a shift registercomprising flip-flops 1066, 1068, 1070, and 1072. A load signal forperforming a parallel data load of the shift register is provided to theshift register at node 1054. A transmit clock signal is provided to aclock input of the shift register at node 1056. Nodes 1058, 1060, 1062,and 1064 are coupled to parallel load data inputs of flip-flops 1066,1068, 1070, and 1072, respectively.

Node 1074 provides a signal taken from the serial data output of theshift register at the output of flip-flop 1072 to an input ofmultiplexer 1076, to an input of XOR gate 1084, and to an input ofmultiplexer 1096. Node 1082 is taken from a data output of flip-flop1066 and coupled to an input of XOR gate 1084. XOR gate 1084 provides anoutput at node 1086, which is coupled to an input of flip-flop 1038.

A transmit clock signal is provided to a clock input of flip-flop 1038via node 1037. A PRBS test input signal is provided to an input offlip-flop 1038 via node 1049. An output of flip-flop 1038 is coupled toan input of multiplexer 1045 via node 1039. A fixed logic zero input iscoupled to an input of multiplexer 1045 at node 1041. A PRBS test signalis coupled to a selection input of multiplexer 1045 via node 1043. Aroll test signal is coupled to a selection input of multiplexer 1045 vianode 1047. An output of multiplexer 1045 is coupled to node 1001.

A transmit clock signal is provided to multiplexer 1026 via node 1025.The output of multiplexer of 1026 is coupled via node 1030 to an inputof output driver 1027. A transmit clock signal is coupled to an input ofmultiplexer of 1076 via node 1075. The output of multiplexer 1076 iscoupled to output driver 1027 via node 1080. Output driver 1027 providesan output at node 1028, which is coupled to pad 1029.

FIG. 11 is a logic diagram illustrating a prior art quad signaling levelreceive circuit. Pad 1101 is coupled to node 1102, which is coupled toeven receiver 1105 and to odd receiver 1106. A receive clock signal isprovided to even receiver 1105 at node 1103 and to odd receiver 1106 atnode 1104.

The most significant bits (MSB) from even receiver 1105 are passed to aninput of a shift register comprising flip-flops 1163, 1165, 1167, and1169 via node 1157. A receive clock signal is provided to a clock inputof the shift register via node 1159. An unload signal for providingparallel data outputs from the shift register is applied to the shiftregister via node 1161. Parallel data are provided at parallel dataoutputs 1171, 1173, 1175, and 1177 of flip-flops 1163, 1165, 1167, and1169, respectively.

The least significant bits (LSB) from even receiver 1105 are passed toan input of a shift register comprising flip-flops 1113, 1115, 1117, and1119 via node 1107. A receive clock signal is provided to a clock inputof the shift register via node 1109. An unload signal for providingparallel data outputs from the shift register is applied to the shiftregister via node 1111. Parallel data are provided at parallel dataoutputs 1121, 1123, 1125, and 1127 of flip-flops 1113, 1115, 1117, and1119, respectively.

The most significant bits (MSB) from odd receiver 1106 are passed to aninput of a shift register comprising flip-flops 1164, 1166, 1168, and1170 via node 1158. A receive clock signal is provided to a clock inputof the shift register via node 1160. An unload signal for providingparallel data outputs from the shift register is applied to the shiftregister via node 1162. Parallel data are provided at parallel dataoutputs 1172, 1174, 1176, and 1178 of flip-flops 1164, 1166, 1168, and1170, respectively.

The least significant bits (LSB) from even receiver 1106 are passed toan input of a shift register comprising flip-flops 1114, 1116, 1118, and1120 via node 1108. A receive clock signal is provided to a clock inputof the shift register via node 1110. An unload signal for providingparallel data outputs from the shift register is applied to the shiftregister via node 1112. Parallel data are provided at parallel dataoutputs 1122, 1124, 1126, and 1128 of flip-flops 1114, 1116, 1118, and1120, respectively.

FIG. 12 is a logic diagram illustrating a quad signaling level receivecircuit capable of operating in a PRBS test mode in accordance with anembodiment of the present disclosure. Pad 1201 is coupled to node 1202,which is coupled to most significant bits (MSB) receiver 1205 and leastsignificant bits (LSB) receiver 1206. A receive clock signal is providedto MSB receiver 1205 at node 1203 and to LSB receiver 1206 at node 1204.

Even-numbered bits from MSB receiver 1205 are passed to an input ofmultiplexer 1293 and to an input of XOR gate 1297 via node 1257. Theoutput of multiplexer 1293 is coupled to an input of XOR gate 1297 andto a serial data input of a shift register comprising flip-flops 1263,1265, 1267, and 1269 via node 1295. An output of XOR gate 1297 isprovided at node 1299. A receive clock signal is provided to a clockinput of the shift register via node 1259. An unload signal forproviding parallel data outputs from the shift register is applied tothe shift register via node 1261. Parallel data are provided at paralleldata outputs 1271, 1273, 1275, and 1277 of flip-flops 1263, 1265, 1267,and 1269, respectively.

An output of flip-flop 1269 is coupled to an input of XOR gate 1283 vianode 1279. An output of flip-flop 1263 is coupled to an input of XORgate 1283 via node 1281. An output of XOR gate 1283 is coupled to aninput of multiplexer 1243 via node 1285. A PRBS test signal is appliedto a selection input 1241 of multiplexer 1243.

Odd-numbered bits from MSB receiver 1205 are passed to an input ofmultiplexer 1243 and to an input of XOR gate 1247 via node 1207. Theoutput of multiplexer 1243 is coupled to an input of XOR gate 1247 andto the serial data input of a shift register comprising flip-flops 1213,1215, 1217, and 1219 via node 1245. An output of XOR gate 1247 isprovided at node 1249. A receive clock signal is provided to a clockinput of the shift register via node 1209. An unload signal forproviding parallel data outputs from the shift register is applied tothe shift register via node 1211. Parallel data are provided at paralleldata outputs 1221, 1223, 1225, and 1227 of flip-flops 1213, 1215, 1217,and 1219, respectively.

An output of flip-flop 1219 is coupled to an input of XOR gate 1233 vianode 1229. An output of flip-flop 1213 is coupled to an input of XORgate 1233 via node 1231. An output of XOR gate 1233 is coupled to aninput of multiplexer 1294 via node 1235. A PRBS test signal is appliedto a selection input 1292 of multiplexer 1294.

Even-numbered bits from LSB receiver 1206 are passed to an input ofmultiplexer 1294 and to an input of XOR gate 1298 via node 1258. Theoutput of multiplexer 1294 is coupled to an input of XOR gate 1298 andto the serial data input of a shift register comprising flip-flops 1264,1266, 1268, and 1270 via node 1296. An output of XOR gate 1298 isprovided at node 1200. A receive clock signal is provided to a clockinput of the shift register via node 1260. An unload signal forproviding parallel data outputs from the shift register is applied tothe shift register via node 1262. Parallel data are provided at paralleldata outputs 1272, 1274, 1276, and 1278 of flip-flops 1264, 1266, 1268,and 1270, respectively.

An output of flip-flop 1270 is coupled to an input of XOR gate 1284 vianode 1280. An output of flip-flop 1264 is coupled to an input of XORgate 1284 via node 1282. An output of XOR gate 1284 is coupled to aninput of multiplexer 1244 via node 1286. A PRBS test signal is appliedto a selection input 1242 of multiplexer 1244.

Odd-numbered bits from LSB receiver 1206 are passed to an input ofmultiplexer 1244 and to an input of XOR gate 1248 via node 1208. Theoutput of multiplexer 1244 is coupled to an input of XOR gate 1248 andto the serial data input of a shift register comprising flip-flops 1214,1216, 1218, and 1220 via node 1246. An output of XOR gate 1248 isprovided at node 1250. A receive clock signal is provided to a clockinput of the shift register via node 1210. An unload signal forproviding parallel data outputs from the shift register is applied tothe shift register via node 1212. Parallel data are provided at paralleldata outputs 1222, 1224, 1226, and 1228 of flip-flops 1214, 1216, 1218,and 1220, respectively.

An output of flip-flop 1220 is coupled to an input of XOR gate 1234 vianode 1230. An output of flip-flop 1214 is coupled to an input of XORgate 1234 via node 1232. An output of XOR gate 1234 is coupled to aninput of flip-flop 1238 via node 1236. A receive clock signal is appliedto a clock input 1237 of flip-flop 1238. An output of flip-flop 1238 iscoupled to an input of multiplexer 1293 via node 1239. A PRBS testsignal is applied to a selection input 1291 of multiplexer 1293.

FIG. 13 is a logic diagram illustrating a quad signaling level receivecircuit capable of operating in a PRBS test mode and a roll test mode inaccordance with an embodiment of the present disclosure. Pad 1301 iscoupled to node 1302, which is coupled to most significant bits (MSB)receiver 1305 and least significant bits (LSB) receiver 1306. A receiveclock signal is provided to MSB receiver 1305 at node 1303 and to LSBreceiver 1306 at node 1304.

Even-numbered bits from MSB receiver 1305 are passed to an input ofmultiplexer 1393 and to an input of XOR gate 1397 via node 1357. Theoutput of multiplexer 1393 is coupled to an input of XOR gate 1397 andto a serial data input of a shift register comprising flip-flops 1363,1365, 1367, and 1369 via node 1395. An output of XOR gate 1397 isprovided at node 1399. A receive clock signal is provided to a clockinput of the shift register via node 1359. An unload signal forproviding parallel data outputs from the shift register is applied tothe shift register via node 1361. Parallel data are provided at paralleldata outputs 1371, 1373, 1375, and 1377 of flip-flops 1363, 1365, 1367,and 1369, respectively.

An output of flip-flop 1369 is coupled to an input of XOR gate 1383 andto an input of multiplexer 1393 via node 1379. An output of flip-flop1363 is coupled to an input of XOR gate 1383 via node 1381. An output ofXOR gate 1383 is coupled to an input of multiplexer 1343 via node 1385.A PRBS test signal is applied to a selection input 1341 of multiplexer1343. A roll test signal is applied to a selection input 1387 ofmultiplexer 1343.

Odd-numbered bits from MSB receiver 1305 are passed to an input ofmultiplexer 1343 and to an input of XOR gate 1347 via node 1307. Theoutput of multiplexer 1343 is coupled to an input of XOR gate 1347 andto the serial data input of a shift register comprising flip-flops 1313,1315, 1317, and 1319 via node 1345. An output of XOR gate 1347 isprovided at node 1349. A receive clock signal is provided to a clockinput of the shift register via node 1309. An unload signal forproviding parallel data outputs from the shift register is applied tothe shift register via node 1311. Parallel data are provided at paralleldata outputs 1321, 1323, 1325, and 1327 of flip-flops 1313, 1315, 1317,and 1319, respectively.

An output of flip-flop 1319 is coupled to an input of XOR gate 1333 andto an input of multiplexer 1343 via node 1329. An output of flip-flop1313 is coupled to an input of XOR gate 1333 via node 1331. An output ofXOR gate 1333 is coupled to an input of multiplexer 1394 via node 1335.A PRBS test signal is applied to a selection input 1392 of multiplexer1394. A roll test signal is applied to a selection input 1390 ofmultiplexer 1394.

Even-numbered bits from LSB receiver 1306 are passed to an input ofmultiplexer 1394 and to an input of XOR gate 1398 via node 1358. Theoutput of multiplexer 1394 is coupled to an input of XOR gate 1398 andto the serial data input of a shift register comprising flip-flops 1364,1366, 1368, and 1370 via node 1396. An output of XOR gate 1398 isprovided at node 1300. A receive clock signal is provided to a clockinput of the shift register via node 1360. An unload signal forproviding parallel data outputs from the shift register is applied tothe shift register via node 1362. Parallel data are provided at paralleldata outputs 1372, 1374, 1376, and 1378 of flip-flops 1364, 1366, 1368,and 1370, respectively.

An output of flip-flop 1370 is coupled to an input of XOR gate 1384 andto an input of multiplexer 1394 via node 1380. An output of flip-flop1364 is coupled to an input of XOR gate 1384 via node 1382. An output ofXOR gate 1384 is coupled to an input of multiplexer 1344 via node 1386.A PRBS test signal is applied to a selection input 1342 of multiplexer1344. A roll test signal is applied to a selection input 1388 ofmultiplexer 1344.

Odd-numbered bits from LSB receiver 1306 are passed to an input ofmultiplexer 1344 and to an input of XOR gate 1348 via node 1308. Theoutput of multiplexer 1344 is coupled to an input of XOR gate 1348 andto the serial data input of a shift register comprising flip-flops 1314,1316, 1318, and 1320 via node 1346. An output of XOR gate 1348 isprovided at node 1350. A receive clock signal is provided to a clockinput of the shift register via node 1310. An unload signal forproviding parallel data outputs from the shift register is applied tothe shift register via node 1312. Parallel data are provided at paralleldata outputs 1322, 1324, 1326, and 1328 of flip-flops 1314, 1316, 1318,and 1320, respectively.

An output of flip-flop 1320 is coupled to an input of XOR gate 1334 andto an input of multiplexer 1344 via node 1330. An output of flip-flop1314 is coupled to an input of XOR gate 1334 via node 1332. An output ofXOR gate 1334 is coupled to an input of flip-flop 1338 via node 1336. Areceive clock signal is applied to a clock input 1337 of flip-flop 1338.An output of flip-flop 1338 is coupled to an input of multiplexer 1393via node 1339. A PRBS test signal is applied to a selection input 1391of multiplexer 1393. A roll test signal is applied to a selection input1389 of multiplexer 1393.

While embodiments of the present disclosure have been described inreference to signaling systems generally, it should be understood thatthe present disclosure may be applied to various types of signalingsystems in various contexts. As an example, the present disclosure maybe implemented in a signaling system existing within a memory system.For example, an embodiment of the present disclosure may be providedwhere either or both of the transmit circuit and the receive circuit areincorporated in either or both of a memory controller and a memorydevice. Thus, signaling relating to memory operations within the memorysystem may be evaluated and optimized. The term signaling is understoodto be broadly applicable. Even within the specific context of a memorysystem, signaling is understood to refer to any type of signals that mayexist, for example, address signals, control signals, and/or datasignals. The present disclosure may be applied to evaluate and optimizeeither or both of memory read operations and memory write operations.

Accordingly, a method and apparatus for evaluating and optimizing asignaling system has been described. It should be understood that theimplementation of other variations and modifications of the presentdisclosure in its various aspects will be apparent to those of ordinaryskill in the art, and that the present disclosure is not limited by thespecific embodiments described. It is therefore contemplated to cover bythe present disclosure, any and all modifications, variations, orequivalents that fall within the spirit and scope of the basicunderlying principles disclosed and claimed herein.

1. (canceled)
 2. A first integrated circuit adapted to communicate adifferential signal to a second integrated circuit over a transmissionmedium, the first integrated circuit comprising: a transmit circuitoperable to output a data signal and its complement as the differentialsignal over the transmission medium in a first mode of operation; andthe transmit circuit further including circuitry operable to generate arepeating pattern signal and its complement for output as thedifferential signal in a second mode of operation; wherein, the transmitcircuit is characterized by a parameter operable to affect reception ofthe differential signal by the second integrated circuit, in the secondmode of operation, the first integrated circuit is operable to vary theparameter within a set of values over multiple iterations of therepeating pattern signal to identify, in dependence on a signal from thesecond integrated circuit, a particular value for the parameter thatoptimizes a desired signaling characteristic, and in the first mode ofoperation, the transmit circuit is operable to apply the particularvalue as the parameter.
 3. The first integrated circuit of claim 2,wherein the signal from the second integrated circuit is dependent onsampling of the differential signal by the second integrated circuit andwherein, in the second mode of operation, the first integrated circuitis operable to identify the particular value in dependence on variationthe sampling of the differential signal by the second integratedcircuit.
 4. The first integrated circuit of claim 2, wherein thetransmit circuit includes a data storage element that is operable toreceive input data and that is output the data signal as a serialsignal.
 5. The first integrated circuit of claim 2, wherein theparticular value is necessarily one of the values in the set of values.6. The first integrated circuit of claim 2, wherein the first integratedcircuit is operable to interpolate an intermediate value between twovalues in the set of values, and is operable to identify theintermediate value as the particular value.
 7. The first integratedcircuit of claim 2, wherein the parameter is a timing reference.
 8. Thefirst integrated circuit of claim 2, wherein the parameter is a voltagereference.
 9. The first integrated circuit of claim 2, wherein theparameter is one of an equalization coefficient, a crosstalkcancellation coefficient, an output drive level, a termination setting,or a transmit clock offset.
 10. The first integrated circuit of claim 2,wherein the repeating pattern signal repeats a pattern that is apseudorandom bit sequence (PRBS).
 11. The first integrated circuit ofclaim 2, wherein the parameter is an offset to an exclusive one of therepeating pattern signal or its complement.
 12. The first integratedcircuit of claim 2, wherein the transmit circuit comprises a datastorage element operable to receive input data, and wherein thecircuitry operable to generate the repeating pattern configures the datastorage element in the second mode to at least partially generate therepeating pattern signal.
 13. The first integrated circuit of claim 12,wherein the data storage element is operable to receive parallel datavia a parallel load input and is operable to output the parallel data asa serial output for transmission to the second integrated circuit deviceas the data signal and its complement.
 14. The first integrated circuitof claim 12, wherein the data storage element comprises a shiftregister, and wherein the circuitry operable to generate the repeatingpattern is operable to configure the shift register as a linear feedbackshift register in the second mode, to generate a pattern of therepeating pattern signal to have a greater bit length than a storagecapacity of the shift register.
 15. The first integrated circuit ofclaim 14, wherein the shift register is a first shift register and thefirst integrated circuit further comprises a second shift register, thefirst and second shift registers operable to interleave even and oddbits for transmission of the data signal in the first mode of operation.16. The first integrated circuit of claim 12, wherein the data storageelement is operable to generate a pattern of the receiving patternsignal in a manner seeded by data in the data storage element followingentry into the second mode.
 17. The first integrated circuit of claim 2,wherein in the second mode, the integrated circuit is operable to sweepthe parameter through the set of values over multiple iterations of therepeating pattern signal to identify the particular value for theparameter.
 18. The first integrated circuit of claim 2, wherein theparameter is a first parameter, and where: the transmit circuit ischaracterized by a second parameter operable to affect reception of thedifferential signal by the second integrated circuit; in the second modeof operation, the first integrated circuit is operable to vary thesecond parameter within a set of values over multiple iterations of therepeating pattern signal to identify, in dependence on a signal from thesecond integrated circuit, a particular value for the second parameterthat optimizes a desired signaling characteristic; and in the first modeof operation, the transmit circuit is also operable to apply theparticular value for the second parameter as the second parameter. 19.The first integrated circuit of claim 18, wherein the first integratedcircuit is sequentially identify the particular value for the firstparameter and subsequently, the particular value of the secondparameter, over multiple iterations of the repeating pattern signal. 20.A first integrated circuit adapted to communicate a differential signalto a second integrated circuit over a transmission medium, the firstintegrated circuit comprising: a transmit circuit; a data storageelement operable to store data for sequential output as a data signal;and circuitry operable to generate a test signal having a pattern thatis repeated over multiple iterations; wherein during a second mode ofoperation, the transmit circuit is operable to output to thetransmission medium the test signal and its complement as thedifferential signal while varying a parameter operable to affectreception of the transmit data output signal by the second integratedcircuit amongst multiple values in a set, and during the first mode ofoperation, the transmit circuit is operable to output to thetransmission medium the data signal and its complement as thedifferential signal using a particular value of the parameter identifiedas a function of a signal received from the second integrated circuitduring the second mode of operation.
 21. The first integrated circuit ofclaim 20, wherein the signal received from the second integrated circuitduring the second mode of operation is dependent on signal quality ofthe differential signal as received by the second integrated circuit,and wherein the first integrated circuit is operable to select theparticular value of the parameter in dependence on signal quality of thedifferential signal as received by the second integrated circuit foreach of the multiple values in the set.
 22. The first integrated circuitof claim 20, wherein the data storage element is operable to receiveparallel input data and output the parallel data as a serial signal. 24.The first integrated circuit of claim 20, wherein the parameter is atiming reference.
 25. The first integrated circuit of claim 20, whereinthe parameter is a voltage reference.
 26. The first integrated circuitof claim 20, wherein the parameter is one of an equalizationcoefficient, a crosstalk cancellation coefficient, an output drivelevel, a termination setting, or a transmit clock offset.
 27. The firstintegrated circuit of claim 20, wherein the pattern is a pseudorandombit sequence (PRBS).
 28. The first integrated circuit of claim 20,wherein the transmit circuit comprises circuitry operable to configurethe data storage element to at least partially generate the repeatingpattern signal in the second mode.
 29. The first integrated circuit ofclaim 28, wherein the data storage element comprises a shift register,and wherein the circuitry is operable to configure the shift register asa linear feedback shift register in the second mode, to generate apattern of the repeating pattern signal to have a greater bit lengththan a storage capacity of the shift register.
 30. The first integratedcircuit of claim 28, wherein the shift register is a first shiftregister and the first integrated circuit further comprises a secondshift register, the first and second shift registers operable tointerleave even and odd bits for transmission of the data signal in thefirst mode of operation.
 31. The first integrated circuit of claim 20,wherein in the second mode, the integrated circuit is operable to sweepthe parameter through the set of values over multiple repetitions of thepattern in the test signal to identify the particular value for theparameter.
 32. The first integrated circuit of claim 20, wherein theparameter is a first parameter, and where: the transmit circuit ischaracterized by a second parameter operable to affect reception of thedifferential signal by the second integrated circuit; during the secondmode of operation, the integrated circuit is operable to vary the secondparameter within a set of values over multiple repetitions of thepattern of the test signal to identify, in dependence a signal from thesecond integrated circuit, a particular value for the second parameterthat optimizes a desired signaling characteristic; and during the firstmode of operation, the transmit circuit is also operable to apply theparticular value for the second parameter as the second parameter.
 33. Amethod of selecting a parameter for use by a first integrated circuit intransmitting data to a second integrated circuit over a transmissionmedium, the parameter operable to affect reception of the differentialsignal by the second integrated circuit, the method comprising: in afirst mode of operation, using a transmit circuit to transmit input datato the second integrated circuit as a differential output signal; in asecond mode of operation, using the transmit circuit to transmit asignal having a repeating pattern and its complement to the secondintegrated circuit as the differential output signal while varying theparameter amongst multiple values in a set; based on a signal from thesecond integrated circuit, selecting a particular value of theparameter; and applying the particular value of the parameter in thefirst mode of operation.
 34. The method of claim 33, wherein the signalfrom the second integrated circuit represents signal quality of thedifferential signal as sampled at the second integrated circuit, andwherein selecting the particular value includes selecting the particularvalue to optimize the quality of the differential signal as sampled atthe second integrated circuit.